Re: the Mentioning in multiple communities, PieFed does the same. Preventing spam is a nice side-effect, but the main reason is to handle the same activities being sent multiple times. The most reliable way to know if you've already received the activity for a post is to have a unique constraint on the DB field for the post's original URL (so once you've created the post for one community, you can't then create the same post for another community).

I was experimenting with adding other people's mastodon posts to a local PieFed community earlier. It's technically fine, not much different than another Mastodon user boosting the post. The slight difficulty is in getting Lemmy instances to accept them - you have to break their Group spec a bit to get it work. It might be worth doing anyways.