If their target audience is disgruntled Firefox users that makes a ton of sense. I would not consider replacing Firefox with a browser based on Chrome/Chromium. It's not that I think it a bad rendering engine, it not, but I don't like the mono-culture that has been promoted and would like to avoid contributing to it, if I can.
No it wouldn't. Even the OpenSSH project has states that they'd prefer that more SSH implementation where around, due to security concerns. Bugs in OpenSSL where/are a serious issue, because of it was almost a monopoly until HeartBleed.
Having a single rendering engine be 95% of the market is not a good option in terms of overall security for the internet.
The rendering engine in Chrome is Blink, which is a fork of WebKit. Safari, GNOME Web and DuckDuckGos macOS browser still uses WebKit. Blink and WebKit is going to share some of the same issues, as they come from the same codebase, but they are two separate rendering engines at this point.
Compared to Brave in what terms? Speed, not sure, but Chromium is known to be better. As far as I know, Brave doesn't allow split tabs or workspaces though.
Not sure about workspaces, is it like profiles?