It's more of a branch than an actual fork. It's using an automated build process where they add/tweak/enable/disable stuff on top.

The analogy I like to use is they (Chromium-based browsers) are in another lane on the same road whereas a complete fork diverges to their own road. The former is still beholden to whatever direction the Chrome road goes.

Yeah, at some point Google may just straight up remove the Manifest v2 code from Chromium rather than just disable it. At that point it can get harder for Brave. Happened to them with eg. mobile slideshow tabs. Google disabled it, Brave kept the feature alive until Google removed it from the codebase.

I'm not familiar with Brave development, but I assumed they are mostly using the off-the-shelf upstream engine and putting their own browser (chrome, UX, etc.) around it.