Yeah, it might have been "fine" at the normal web browsing part, but Microsoft kept trying to push their proprietary extension store. Also, didn't they not support extensions for the longest time? I think that was the biggest reason they switched to chromium, so they could use all those existing chrome extensions?

Yes but it was changed quite a bit, it supported way more standards and was getting way more updates to keep it up to date. The issue is that was expensive and also people complained that it some websites didn't work on it, so it made more economical sense to switch it to chromium. I really wish they had kept it though.

Sorta, sorta not. Looking at the wiki page, it used "EdgeHTML" as the browser engine, which was a fork of ie's engine (MSHTML). But it was a massive overhaul removing a bunch of legacy code and rewriting parts to fit modern standards and to make it compatible with webkit. It was maintained alongside ie11.

I remember testing it out and it being a lot faster than ie was when it first came out, but I've always been a ff user so I didn't switch to it.