Meant to comment this earlier. On your last point so far as I’m aware there’s currently no way to create a link post (direct URL lemmy link as you say) from Mastodon/microblog to Lemmy. The reason your test post is linking back to the Mastodon instance is because of the image attachment, because you can create image posts between the two.

If you drop the image attachment, while it won’t look as nice, you can get the separate title, link, and body text without it looking too bad. Unfortunately it will lose the visual draw in the process, but that seems to be the workaround for the time being.