Dear ziproot,
Really interesting questions and about a wonderful game too! I'll do my best to
answer.
CC-BY-SA 3.0 and CC-BY-SA 4.0 are known as 'GPL-compatible' because they don't
add additional restrictions on the end user. It is fine to include them with a
GPLv3 piece of software and they are allowed in Debian.
The Unsplash License appears not to be allowed in Debian ("License must not be
specific to Debian" in the Debian Free Software Guidelines), because it does
not allow the use of the copyrighted work in any way that competes with
Unsplash. I believe that, unfortunately, these specific game assets can not be
distributed by Debian and would have to be replaced with free and open source
alternatives.
Including AppImage binaries that are themselves made from FOSS isn't a problem
for the GPL, and you can also often include proprietary binaries alongside the
GPL with the 'mere aggregation' principle that you mention. The bigger issue is
that Debian tries to build everything from source - I think you'd need to be
generating the AppImage from source in your own build scripts to be accepted
into Debian.
You mention the non-free and contrib sections. Technically, these are not part
of Debian. I don't have any first-hand knowledge of these so I'll defer to
someone more experienced to answer your questions about those!
Best wishes,
Sebastian
