Per Xiaodi's request, I'm going to reply here but ask future questions to go to the review thread.
The implementation essentially depends on that proposal to be able to handle custom executors.
Right. If we end up with a custom executor that doesn't support isIsolatingCurrentContext(), we're somewhat stuck with as?: we could let it pass (that's a data race), possibly with a runtime warning about what's happening, or we could reject it and make conformances isolated to a global actor with a -pre-isIsolatingCurrentContext() executor unusable.
Yes, that's fine. Nonisolated conformances can be used from anywhere (just like nonisolated everything else).
Doug