What I have working now suits my purposes, and if there is community
interest, I would submit a patch along these lines, also incorporating
further comments you may have.
I appreciate your work and use case, though I do not currently think
this is something I want (another SSH connection plugin) in core.
Thankfully, the connection plugins are *very* tiny, and it's pluggable
for exactly this reason.
The connections don't change very often, so you should not have any
trouble at all with this approach.
Now, this is a ginormously long thread, so I'm not really sure what
it's about anymore, and responses are also long.
If it turns out we can't set ansible_ssh_user via inventory variables
and make it actually /use/ that username, I believe that is something
we need to fix.