a) why it is ethanol that seems to be the drug of choice for many cultures (as opposed to other substances, in the time the plentiful and complex methods of creating alcohol were developed, a way of safely preparing other stimulants like mushrooms probably could have been found)

The main reason that ethanol is humanity's psychoactive drug of choice is that it is ubiquitous. Wine quite literally makes itself and once you get a yeast colony going in habituated area beer is quite easy to make. Same with honey and cider. Its effects are also generally quite pleasant and comparably mild. The majority of the other psychoactive drugs used in modern times are the result of very complex and sophisticated acts of chemistry and simply weren't available to humans. The non-alcohol drugs that could be found naturally such as mescaline, DMT, psilocybin and so forth are intense experiences with pretty seriously unpleasant side effects including delirium and vomiting. They generally would be used in a shamanistic or mystical context and not recreationally for pleasant psychoactive effects. Cannabis sativa/indica are obviously exceptions and were used quite widely but its growing range is nothing compared to the nearly world-wide easy production of alcohol.

b) and why it became used in first place. Alcohol has many obvious drawbacks and still humanity has spend loads of resources to create it. Why did societies that still had to struggle at times spend their time at something like that?

People drink alcohol because they like it's effects. Its really just that simple. Drunkenness was considered bad form through antiquity and the middle ages up through modern times but enough people indulged moderately without too many ill effects that it's prohibition would be very problematic because of its popularity. Some Christian and Islamic countries have had alcohol prohibition but a huge black market results because people will still continue to drink.

in the time the plentiful and complex methods of creating alcohol were developed, a way of safely preparing other stimulants like mushrooms probably could have been found

I'll just take on this part. First, neither psychedelic mushrooms or ethanol are stimulants.

Second, basic ethanol production through fermentation requires no special materials, just a source of sugars or starch and yeast. If you can make bread, you can make beer. Brewing is very old.

What obvious drawbacks are you referring to?