If you pass a file as parameter to apt install, like:
apt install ./package.deb
It will work, at least on buster.
A partial "mea culpa" ;/
I was completely focused on Debian tools and never thought of searching
the whole web.
My gut feeling is still there should be a way to use Debian tools.
It would be nice if this was documented on the apt manpage, i have a strong feeling that it's just a matter of opening a bug report asking for it, or even better, sending a patch to the maintainers. Whatever suits you better.