After watching this, I decided to try and fix the shotgun problem in the spare time I had tonight.
Unfortunately, it still results in the shotgun being invisible and now I've ran out of time, but at least the disguise no longer shows the Spy's weapons.
I'm a bit baffled at how this still wasn't fixed. At least, in plugin form.
If I missed out on something then sorry and dang, all that effort for nothing.
1/13/2021 7:05 PM EDIT: Updated
ShotgunDisguiseFix.sp, engineer disguise shotguns now function and show properly. Other disguises are untested because bots on
bot_mimic can't switch to sappers.
1/13/2021 11:54 PM EDIT: Decided to add debug command to remove the disguised weapons, shotguns are confirmed to be working for the other disguises. Hooray!