You’re completely right that there are likely to be major scalability issues, at this point I don’t think anybody fully knows what the implications are, and it’s not getting a lot of discussion. This is part of why Meta’s proceeding slowly and presumably we’ll see a lot of performance work over the next few months to deal with the expected onslaught.
Right, a post embed that results in anybody visiting your site gets tracked by Meta (whether or not they have an account there).