Every single question you've posed has already been answered in this forum many, many times over the past few weeks. A bit of simple searching would have revealed this.
Yes, Animate is just Flash's new name.
No, you don't have to stop using Edge if you don't want to. It's no longer being developed, but it still works just fine.
Yes, of course your test works in a non-Flash browser, because you created a Canvas document. Working without the Flash plugin is the entire point of Canvas mode (which has been in Flash since 2013).
Yes, it's possible to stretch the canvas to fill the browser window. There are many threads here discussing it.
You're right - as an experienced forum user I should have been a lot more diligent before posting.
Originally I came to the Forum to ask a question about Edge Animate ... then found the big long "discontinued" thread and posted here as a bit of a knee-jerk.
I was a massive Flash nerd from 1998-2006 but thought that Steve Jobs killed it and I never really kept up with the news.
I got the latest version of Animate and was delighted to see how many keyboard shortcuts still work (or which ones I still remember).
Anyway, I'll go search for ways to make the content scale better on mobile screens.