Yes and no:

Yes, votes are shared between instances. However, they aren’t synchronized. When you click on “upvote”, your server sends a message to the community with the requested change. The remote server applies that change, then sends messages to other remote servers with that update.

There are retries built in, but if for some reason one of those messages doesn’t get delivered, the different instances will get out of sync. I do not think there is a mechanism for “catching up” once this happens.

Yes! Some instances have downvotes disabled, they will be discarded even when you or I downvote something that’s on their server.