When it comes to /kbin as a platform, the federation of downvotes will certainly work and will be configurable per instance. Today, I started the initial work on implementing new ActivityPub services from scratch. This is a good time to start a discussion on how it should work on kbin.social - to your points, I would also add:
Downvote federation only applies to remote threads from other instances, without affecting local therads
ThatOneKirbyMain2568