The official web client works very well as a PWA on mobile.
The official native apps are also an option, but are controversial for a number of reasons. I don’t use it, but from what I’ve heard Tusky is a very popular alternative.
The PR hadn’t been reviewed and was merged by mistake, from my understanding.
Or does a self hosted lemmy instance federate all comments from all threads once it knows about a server?
Only in subscribed communities, but yes.
On the other hand, I don’t really trust Reddit with my upvotes/downvotes all that much more than random users, so I already refrain from voting on content I wouldn’t want to be associated with…
Of course, Reddit can still see what posts I view, while that isn’t the case for Lemmy (at least since I self-host an instance).
I believe this is the number displayed if you view the community on the original instance?