Culinary history is a hobby field for me, so this won't be as detailed as I'd like, but I can give a little insight into the proliferation of foods that look so bizarre and unappetizing to us today.

Addressing your points here separately:

What's with all the Jell-O? This is a more complicated subject than it appears. The answer is a witch's brew of social change, new technology, and that traditional bugbear, class.

We're used to thinking of gelatin as: a). a school lunch dessert with ersatz whipped cream on top: b). something you'll find in inexpensive buffet and cafeteria restaurants, or: c). a dish that grandparents or an elderly aunt habitually bring to holiday parties and are then mostly obliged to bring home. From our perspective in the 21st century, it's okay but not super sophisticated fare.

However, gelatin has a much ritzier history than this would suggest. If you don't have the option of running to the supermarket and grabbing a box of Jell-O or Knox (and of course, pre-19th century people didn't), gelatin is actually a pain in the ass to produce. Food scientists will tell you that gelatin is collagen that undergoes partial hydrolysis. A pre-19th century cook would tell you that gelatin is the result of boiling the hell out of a bunch of bones and skin and then straining and chilling the result. And that's when your problems really started:

• If you couldn't keep it chilled, it would simply melt, which was no small matter in the era before refrigeration.
• You also had to serve gelatin on chilled plates (or else just serve it very quickly) for the same reason.
• Younger animals have more abundant collagen than older animals, which meant sacrificing the economic potential from an adult animal in order to use a younger version of it sooner. (NB: This is one of the reasons that veal stock in French classical cuisine is omnipresent; in addition to the neutral taste, it was and remains the best possible way of introducing the body and mouthfeel of gelatin into a sauce.)
• For the final turn of the screw, you had to have at least some sense of what you were doing. Bones and skin do not come pre-labeled with the amount of gelatin they'll wind up producing, and different species (e.g., chickens, pigs, cows, and fish were all used for savory aspics) give different amounts. Every pre-19th century cook in training inevitably produced aspics that didn't set because the ratio of gelatin to liquid was off, and experience was really the only way to avoid this.

Bottom line: You were not going to find gelatin dishes on the tables of many middle- or lower-class people. The expense, time required, and difficulty of producing it made it the more typical production of dedicated cooks employed by wealthy families, and it was very much an upper-class food as a result.

And then two big things happened. Powdered gelatin (introduced in the mid-19th century) suddenly made its use much easier. Refrigeration (increasingly common in Western homes from the 1930s onwards) had an enormous impact on basically everything to do with home cooking. A lot could be written about both subjects, but for today we'll condense it to the following:

• Gelatin was suddenly inexpensive and accessible to basically everyone.
• Being able to measure and know exactly how much gelatin was going to be in a dish solved the most maddening problem related to cooking with it.
• Anyone with a refrigerator could keep a gelatin dish stable before serving it.

So in a nutshell, this is why you see gelatin dishes start to appear with greater frequency in middle-class cookbooks from the late 19th century onwards, and why that frequency exploded when refrigerators became common items in middle-class homes. As always, when the middle class suddenly gets its hands on something that was previously available only to the rich, you tend to see a lot of creativity with the results, but not all creativity is necessarily going to be, er, awesome.

By the way, if you want a small taste (no pun intended) of how finicky gelatin could be for folks in the pre-19th century, you can try making xiao long bao, or Chinese soup dumplings. Nearly all recipes involve boiling a bunch of pork bones and skin to extract the gelatin, chilling it, cutting it into cubes, and then mixing the cubes with the rest of your filling in a chilled bowl. You can't let the filling get too warm or the gelatin will melt; the idea is to keep it chilled until the dumplings are steamed, at which point the gelatin is converted into a liquid and becomes the soup in the soup dumplings.

Amazing dish. Huge pain in the ass to make.

Both world wars, the ration system, and the growth of advertising in the food industry had their own impact. Food companies distributed pamphlets on how their products could help housewives cope with the frequent shortages of fresh meat and dairy items during both wars. Some of the dishes developed in the early to mid-20th century in these circumstances were legitimately good (e.g., Canadian War Cake, which uses no eggs, butter, or milk). I will be kind to the others and say they reflect the ingenuity of people who were trying to create palatable recipes in difficult times, and who did not always succeed. The use of Jell-O and Knox to stretch meat scraps or augment sauces was common, and the popularity of "congealed salads" (using flavored gelatin to suspend vegetables in a mold) dates to this period as well.

Perhaps the most defining characteristic of the post-WWII period was the practice of buying canned or pre-made foods at the supermarket and then doctoring them at home (e.g., using dried onion soup packets and sour cream for dip, mini-pizzas on English muffins, canned mushroom soup as a thickener/binder/sauce in casseroles). Again, gelatin was a natural fit for this approach to cooking, and both Jell-O and Knox sought to capitalize on it. One of the things you'll notice about really horrifying recipes from yesteryear is how many of them were functionally advertisements for a product that had no business being shoved into as many recipes as it was.

Did people really eat these? Have palates changed? Yes. No. Sort of. Maybe.

This is a bit tough. Obviously, I can't speak for all people and all cultures. What I can tell you is that it's pretty instructive to read through different editions of cookbooks that have been coming out for decades (e.g., The Professional Chef, published by the Culinary Institute of America in multiple editions spanning 1974 to present), or just comparing comprehensive cookbooks that were written in different eras. Recipes get dropped, recipes get added, and each era has its own influence on what we consider "basic cookery." One of the most fascinating patterns is how the Western palate has grown: a). noticeably more adventurous, and: b). less snobby toward non-Western cuisines, although that's been a slow process at best. In Ruth Reichl's book Garlic and Sapphires, which is about her tenure as the New York Times food critic between 1993 and 1999, she commented that she got complaint letters from people when she reviewed anything that wasn't a French or Italian restaurant.

Anyway! Studying cookbooks wouldn't necessarily be a good way to judge what people were really eating, because formal cookbooks tended to eschew the really appalling recipes you'll find in your links here. There were absolutely home cooks out there who clipped these monstrosities from magazines or newspapers and tried them. Again, many of these recipes were poorly-done branding attempts from food companies that people might have been inclined to try with the ingredients on-hand, but not necessarily repeat. Everyone is influenced by the food trends that surround them, but a bad recipe isn't going to be improved just because some of its ingredients are fashionable.

Having said that, some perfectly decent recipes from the 1950s-1970s look horrifying, not because they actually are, but because food photography and presentation in the U.S. were in what we might call the toddler years. You can see this in action at James Lileks' Gallery of Regrettable Food.

Were they generally thought of as insane as we do today?

I present without comment the song Lime Jell-O Marshmallow Cottage Cheese Surprise, written in 1980.

Resources:

• British Food: An Extraordinary Thousand Years of History by Colin Spencer. (I wrote "Five of them good" below the title. Just kidding, Brits; you gave us Heston Blumenthal.)
• Jane and Michael Stern's many books on 20th century American culinary history, food trends, and restaurants, but the most immediately useful would be Square Meals.

This has already been fairly comprehensively answered by /u/Cenodoxus and also by /u/yodatsracist digging up some older answers (appreciate the callout, too!), so I'll just do my usual latecomer thing of picking up a few shiny bits around the edges.

The thing about blending meats and sugars isn't so much a detail of mid-twentieth-century cookery as an odd anomaly in the rest of twentieth-century Western cuisine. That is: we divide our food into savoury and sweet, and we don't expect things to cross that line much. However, if you look at medieval recipes (from almost any culture), you'll see things mixed up. There are recipes for apple pie which include onions, recipes for meat pies which include fruit and sweet spices, and honey in all kinds of stews and soups.

Now, the line isn't as strict in modern cuisine as we think it is. Tomato ketchup (vile stuff that it is), is basically sweet. We put fruit sauces on meat, and sweet glazes on ham. Various cooks swear by including chocolate in chilli (I've never gotten it to work). There are a number of variations on potato dishes in Northern Europe which are sweetened - indeed, in Iceland, there's one where they're literally dredged in brown sugar. In Ireland, you'll see people dipping chips (fries, in American terms) in milkshakes, and almost everyone sweetens their porridge, if they eat it at all, in some way. But these fade into the background a little.

The division between sweet and savoury seems to stem from the Victorian era, and the old reliable Mrs. Beeton, the cause of so much historical disconnect in Western cuisine. Her American counterparts and imitators probably share some of the blame here, admittedly. The bit that I'd point to most are the menus she provides, which separate out sweet dishes from savoury dishes into different courses. There is, of course, a degree to which she's reflecting the practice of the era, but when it comes to a prescriptive/descriptive division, Isabella Beeton falls firmly on the prescriptive side of the fence.

On loaves, these are essentially a descendant of the pie, which has a long, long history. A meatloaf (or whatever-else-loaf) is essentially a pie with no pastry crust. These existed in previous eras - patés, various moulded foods, and so forth as well as steamed puddings - but the 50s obsession with them is essentially a result of technology of the era, which made meat-grinding, standard loaf-tin shapes, and the fats necessary to grease tins and make stuff stick together available in ways they weren't before. This is a subtle change, mind - all these things were available before, but slight increments in their ubiquity all the same time result in meatloaf.

And finally, palates changing: yes, there's some of that too. Some of the medieval dishes are just too rich for us to stomach in any quantity. Medieval mince pies (that is, the sweet sort sold in the British Isles around Christmas) combined meat, dried fruit, and suet, and they're incredibly rich and dense. Green sauce, which is made from chopped herbs and vinegar, and was very much a standard condiment in its time, is rather too strong a taste for us to handle in any but the smallest quantities, and the Roman garum (another sauce/condiment) is, by all accounts, utterly vile to the modern palate. And there are whole areas of medieval Arabic cookery that are much too sour for us, using vinegar and rue in quantities that have led me to investigate if the recipes were translated correctly (they were).

Sources:

C. M. Woolgar, D. Serjeantson, and T. Waldron, Food in Medieval England: Diet and Nutrition

Michael Pollan, The Omnivore's Dilemma: A Natural History of Four Meals

Alfred W. Crosby, The Columbian Exchange: Biological and Cultural Consequences of 1492

Kathryn Hughes, The Short Life and Long Times of Mrs. Beeton

Nawal Nasrallah (trans.), Annals of the Caliphs' Kitchens

You may be interested in these older answers:

• Post-war America saw the promotion of several peculiar dishes based on newly introduced industrial food stuffs (jellied salads, spam loaves, etc.) which seems profoundly unappealing from current standards. How popular were these dishes compared to the industrial staples themselves. by /u/chocolatepot and /u/TheShowIsNotTheShow.

• To what extent were the housewife manuals of the mid 20th century taken seriously? by /u/caffarelli

(More below, but it only pings three people per comment).