Yeah, I’m pretty sure they interpret the endpoints differently so unless a mastodon post is directly targeted at a community or something, lemmy just drops it. That’s why I still run a dedicated mastodon and lemmy instance. Kbin promises best of both worlds but doesn’t seem mature enough for my tastes.
I don’t believe so, it looks like they use their own API. I would guess because they implement a subset of the subsonic API and it might be hard to juggle both. But there’s no harm in trying and seeing what happens.