I'd be wary of getting a conversation node from anybody other than the original author (as described in the second approach).

There's a reason why, if you want to resolve a missing post in Lemmy, etc, you have to use the fedi-link to retrieve it from its source, not just from any other instance that has a copy (because, like the "context owner", they could be lying).

For Group-based apps, conversation backfill is mostly an issue for new instances, who might have a community's posts (from its outbox), but will be missing old comments. Comments can be automatically and recursively retrieved when they are replied to or upvoted by a remote actor, but fetching from the source (as you arguably should do) is complicated by instances closing (there's still loads of comments from feddit.de and kbin.social out there - it will be much worse when lemm.ee disappears). So perhaps Lemmy could also benefit from post authors being considered the trusted owner of any comments they receive.