AFAIK in Mastodon deleted messages don’t be deleted in Discourse. Deletion is restricted action in Discourse. And Discourse isn’t a Mastodon instance. Discourse is using ActivityPub. Different thing.
And every Mastodon user knows, or should know, that deletion is very unreliable. It spreads thru Fediverse or not.
Forum is forum and ActivityPub (practically only Mastodon from Discourse point of view) is its own thing.
By Followers-Only you mean ”Publish Post #1 and deliver it to the followers of the Group Actors” (or normally after 5 mins) for OP? Answer for such topic isn’t going to followers, into global public timeline. It is just matter of which timeline someone sees messages.
It just means it is subscribed feed as normally in Mastodon. Nothing secret.
When someone answers (which happens really rarely because of group actors and how they boosts, but so it works nowadays) to a topic from Mastodon, it is a public message in that forum and in Mastodon, as usual. And if someone from forum answers to that comment, not into topic (thing, that forum users don’t realize) mentioning that Mastodon user, that post is totally public and be federated just normally.
Or did I totally misunderstand your question?
This is different thing and worth of its own topic, but my opinion is that nothing should do or happen just because of some subset wants something, that is beyong normal limit, or they are using theirs own definition. Meaning that nothing above isn’t matter of consent or privacy.