1. Brave, Vivaldi, and Opera have all announced they'd maintain support for Mv2 past Google's deprecation date [1].
2. Your guess is correct - one of Google's stated motivations is to make the extension review process easier and less error-prone; having a way to opt-out would be counterproductive in that regard. I strongly doubt they'd accept the PR upstream; there is a chance other players could maintain patches to modify Mv3 but the effort of designing and implementing a new spec around the Mv3 spec and convincing extensions to maintain yet another platform means this is unlikely to happen in practice. Keeping Mv2 around is a more reasonable approach (and one that is compatible with Firefox, as well).
[1] https://www.zdnet.com/article/opera-brave-vivaldi-to-ignore-...