- Can we see Mastodon posts from Lemmy?
Yes, with some asterisks. Mastodon users see communities as users. for example, they see this community as the @Mastodon@lemmy.ml user. If they tag that user in the toot, then the first line of their toot is the title, second line the link, everything after that is the body. Then it'll show up in that community as a normal post.
- Can we search their hashtags somehow?
No. If you want both link aggregator and microblog functionality, use Kbin. It offers both in one.
- Can they comment on this post from a Mastodon client even though I posted it from wefwef while logged in at lemmy.world?
Yes, as long as they're following the community "user" and see the post. They can then reply to that post, which shows up here as a comment.