All, don’t know. But more platforms, yes.
Even if the answer for that platform is “yes”, could be that Tor won’t start if Whonix is using Tor configuration features only available in a newer Tor version. Maybe %include is unsupported in Debian buster version. I don’t test that since design was decided as per above post and compatible with all platforms not a higher priority goal.
Currently no. But nobody ever noticed for any unsupported platforms. Possible to add code to skip that Tor package from deb.torproject.org download code.