1. I think it can, you just need to give it the machine’s name or local IP and the port.
2. Nope!
3. Caddy handles all certs from LE and renews them for you too. Nothing you need to do on your own.

Is there a way to hook the custom Listenbrainz playlists into Lidarr so I could have a new mix / recommended playlist each week?

Yes this is possible. I have a few hosted items with subdomains and I have it set up as follows:

All subdomains point to the same IP. Router port forwards all 80/8080 traffic to server. I use Caddy in Docker to forward the requests based on the subdomain to the appropriate docker container hosting the actual service.

This makes spinning up something new simple. You get a docker container of New Thing going, edit the CaddyFile to point to it too, set up new subdomain in cloudflare. No new open ports needed.

Hope this helps!