Yes, SGen should be a lot better, but Unity cannot use it because they hold and pass raw pointers around everywhere. That's fine for Boehm but not possible with SGen. They're working on fixing this already but not sure why they aren't planning a move to a better GC.
Yes, but it also puts them in an awkward situation! They recommend (or even require, for some platforms) using IL2CPP for release builds which will still use Boehm GC and not run as quick as CoreCLR.
