For the third and fourth points, I think the comparison to email is apt here. If you do a GDPR data request/removal service on your email provider, it's unreasonable to expect that they chase down all the people you've sent emails to and ask them to delete them.
As far as I know, Lemmy doesn't send any data to other instances unless you explicitly request them to (by either subscribing to a community or sending a message/post).
(Also, I am not a lawyer or expert in GDPR, so don't take this as legal advice!)