>Weren't there a lot more comics in the 50's that were non-capeshit, almost the the point of matching the vaunted diversity in topics of manga?
Pretty much. There were westerns, war comics, horror comics, romance, crime, sports, and drama. There was even an underground scene of smut like Tijuana Bibles. Think hentai doujins but you could get in small trouble with cops back then if found with one that was too obscene.
Still not near the vibrancy of manga's industry or perhaps even the Franco-Belgian one.
>Until, like everything in the US, (im)moral crusaders had to ruin everyone's fun and had the Comics Code Authority shut down everything but capeshit?
That was a big part of what knee-capped comics. Western, military, and horror books were still around into the 70s. What finally killed them were capeshit fanboys who finally got into managerial positions at publishers. They shifted all resources to their favorite capeshit characters, made them with fanboys in mind, and non superhero books were left to wither. At least if people like Darwyn Cooke and Chuck Dixon are to be believed. I am sure that 1970s economic crisis played a large role too.
Old EC's horror anthologies are still great, most do not feel dated (not much anyway), and had consistently great artists working on them. The same can't be said about capeshit comics from the same era. It always makes me wonder what could have been if CCA was not a thing or American creators worked to overthrow it like French did to censorship efforts in their country.