@feditips given the implementation of ActivityPub on Mastodon, my current (limited) understanding is that there is no way to properly answer my question No. 2: A service would need to recursively query who is following whom for a given comment thread and what servers are thusly notified of comments to display them to their users.
And going to the OP post on their server to view the comment thread is not common user behavior, hence we may encounter unwitting reply toxicity. Frustrating.