You can post from Mastodon to Lemmy by putting a mention to the community name (like @community@instance)

Mastodon users can follow the communities or the users.

So yes it pretty much works in both directions.

Yep! You see it from time to time.

@mouseirl this answer is from my mastodon account.

https://merveilles.town/@prahou boosts his comics from Lemmy quite often.