If one instance doesn’t allow creating new communities and I register a lemmy account on that instance am I going to be able to create a new community on another instance using that lemmy account?

No, they’re totally separate websites, and everything you do is local to the website you’re logged into. When you view remote communities and posts, you’re viewing locally stored copies of them, and when you interact with them, your local host sends the updates back to the site hosting the community so the two copies stay in sync.

Also if it has turned off downvotes is it possible to downvote posts on other instances?

Also no. You’re only interacting with the local copy. If you can’t down vote things on the site you’re using, then there’s no down vote action to record and synchronize with the community host.

If you want those features, you’ll need to use another Lemmy-based website that has them enabled.