Sure. Perhaps keep a short explanation why Debian buster (even bulseye) version is Undocumented, Untested or Unsupported Features at Whonix. Perhaps even just one short sentence for better usability and a few details in a footnote. We aren’t coerced to document everything in every version. Only as far as the Contributors and Authorship - Whonix are willing to support it.
Sure.
Even Qubes-Whonix isn’t too complicated. I am demonstrating how to use flatpak in (Qubes-)Whonix on this page:
Chromium
If you would like, what would be useful would be a wiki template flatpak-install.
[1] I actually researched that in context of Dev/Default Browser - Kicksecure but since kicksecure.com isn’t public yet and since flatpak use instructions are interesting (such as for the use case here), I’ve added to Whonix wiki.
Related:
FlatPak as a Software Source / flathub as a source of software