Now, I can't answer all of your questions, but I can say that pizza was a relatively new food in the 1950's, at least outside of Italian ethnic enclaves, and only really becoming ubiquitous in the 1960's and 70's. My favorite example of this is the dialogue from this 1961 Warren Beatty movie called, Splendor in the Grass, which was winner of the Oscar for Best Original Screenplay, and is set in the 20's.

ANGELINA. (Sitting in the other chair.) Look, Bud Stamper...you drink...you get sick. What's the sense of that?

BUD. What kind of a speakeasy is this? You don't want to sell me beer I'll go somewhere else.

ANGELINA. (With a shrug) If you want...but why don't you let me bring you something to eat first? Would you like something to eat? Mama is making pizza in the kitchen. Would you like pizza? Mama makes the best pizza in New Haven.

BUD. Pizza? What's that?

ANGELINA. (Amazed) You never heard of pizza? Where you from?

BUD. Kansas. I'm from Kansas.

ANGELINA. (Disbelief) Kansas?! ... Where's that?

BUD. (Defensively) Kansas is the very heart of the U.S.A.

ANGELINA. (Still not convinced.) But... what is it?

BUD. It's a state.

ANGELINA. A state... And that is your home? What is it like in Kansas?

BUD. (Nostalgically, moved by his memories.) It's...friendly. That's what it's like. It's friendly. And everyone know who you are, and knows your name, and ... (he stops, overcome with loneliness and homesickness).

ANGELINA. (With understanding) You like it better than here, why don't you go back? You must have a sweatheart back there.

BUD. (After a pause.) I did. (He looks away).

ANGELINA. (Tapping his shoulder) Look.. you wait until I go to the kitchen, and I'll bring you some pizza. Okay? (Bud doesn't respond.) The one Mama's making, it's for Mr. Mangolomino...but I'll tell her to make him another one. He can wait. But you... you can't wait. Not for your first pizza. (She rises). Alright? (A smile).

BUD. (Warmed by her friendliness) All right.

I remember reading that the movie was one of the things that played a role in popularizing pizza with a wider audience, though it was clearly already on the upswing. It definitely wasn't the first pop culture reference to pizza, either: Dean Martin famously sang, "When the moon hits your eye like a big pizza pie, that amore," in 1953. Of course, one doesn't need to look past the title to realize the song is marketing to a degree in ethnic kitsch.

This pizza history lists the first American pizzeria as opening in 1905, in New York. The first Chicago style pizza, from Pizzeria Uno, only dates to 1943. However, every pizza history I've read (pizza is great) credits returning World War II G.I.'s coming back from occupying Italy in the late 40's as the thing that pushed pizza out of the Italian enclave and into America's palate. This New York Times piece notes that pizza "did not start receiving regular mentions in The Times until the 1950's." But from there, it quickly became popular: in the 1950's, pizza was even available at some movie theaters, as this commercial demonstrates (if you're curious, the toppings are: cheese, pepperoni, and sausage). This webpage shows a pizza recipe from 1955 aimed at house wives (the default topping is anchovies, other options, which omit mozzarella and just use Parmesan cheese, are "thinly sliced Italian sausage, cooked sausage links, frankfurters, or smoked tongue").

The first frozen pizza, Celentano Brothers, dates from 1957. This history says that the first Pizza Hut was found in Wichita, Kansas in 1958 when two brothers, Dan and Frank Carney, borrowed $600 from their mother to open a restaurant. Like many other fast food restaurants founded in that era (McDonald's, Burger King, KFC, etc.), the chain expanded rapidly, extending into Canada by 1968, no doubt buoyed by the car, suburbanization, relatively large disposable incomes, and other macro-sociological factors. Domino's, similarly, was founded in Michigan in 1960; I don't know about the others, but Domino's delivered from the start, I believe; look at this old ad, when they were stilled called Domi‐Nick's. Domino's was certainly the innovator of the modern pizza box, to the point where the box we know and love as the pizza box was known as a "Michigan style" box. As Pizza Hut and Domino's expanded, they were quickly competing not just with locally owned shops but with other pizza franchises, like Shakey's (founded in Sacramento in 1954), which apparently had 272 pizza parlors by 1967 and opened a Canadian location the next year.

In terms of New York Times coverage, which is probably a leading rather than lagging indicator for national food trends, pizza "first surpassed hamburger in Times coverage in 1976. The two bounced around in close competition for the remainder of the 1970's, but in 1982 they diverged, and pizza never looked back." It's worth mentioning that, while the hot tog only occasionally competed with the hamburger, the taco is now in second place (behind pizza) in terms of all casual foods mentioned in the Grey Lady. So pizza became was popularized in the 1950's, common by the 1960's, everywhere by the 1970's, and seemingly America's favorite (casual) food by the 1980's.

I hope it's okay that as a European I ask this simple comprehension question:

(for example, salami is not a common pizza topping, but pepperoni is)?

What is pepperoni exactly? Isn't it a kind of spicy salami? Here in Europe, salami is a very common pizza ingredient, most Italian restaurants actually have a Pizza Salame on the menu whereas pepperoni is usually saved for a spicy version called something along the lines of Diavolo.

Most pizza places in my part of S.E. Massachusetts have always been Greek, going back to at least the 60's, from my item personal experience. I wonder when they started up? Greek owned pizza places have a slightly thicker crust, lots of olive oil and I think their special sauce spice is Greek oregano.