That’s the idea, but in practice since the data exists independently on each server, it takes network time and computational time for them to align. In practice I expect comments to function as you expect, and upvotes to be slightly off depending on which instance you’re viewing from.

Things get a bit more weird when an instance gets defederated from another instance. My understanding here is that if you have instance A defederate from instance B, but instance B was listening to some of instance A’s communities, that instance B will have an independent replica of that community that doesn’t sync (this happened when beehaw defederated from open registration instances like lemmy.world).

No, they use ActivityPub underneath, so you can go to kbin/mastodon communities/users/etc. from Lemmy and vice versa.