basically limitless
Theres a few things aginst the rules.
And you need to follow the Twitch TOS, ROC etc
See also
And other parts of the extension guide
Yes there is a limit on the size of the zip you can upload, but if get anywhere near that you probably have a problem or need a more optimal way to handle your assets
Sure.
Extensions are “just” websites. So you can do pretty much anything a website can do.