My attempt at a stupid question is: Do you need an account on every instance that hosts a community that you want to interact with (e.g., upvote, post, reply)?
You do not. You can interact with every federated instance and they will see your posts & comments. Sometimes you might not even realise that person you chatted in the comments was from kBin.
Errors with beehaw are expected, since they defederated from us, but we still didn’t defederate back from them. So, in a partial federation situation like this, you will be able to see their posts and interact, but your contributions there would only be visible to other lemmy.world users.