Yes, you can do this from almost anywhere in the Fediverse. As Mastodon does not recognise headings, the first paragraph becomes the heading. This means that it looks better on Lemmy if you put the community tag at the end of the post and start with a content-related introduction.
Sort of, it's because a lot of microblogging software doesn't have the same post format with title/post body. The current workaround for microblog to Lemmy and similar is to format it like this:
This is the case with all content (posts + replies) on Mastodon, as it appears separately in the timeline. In this case, it has nothing to do with Lemmy.