I'd love for him to use a Mac with two monitors and wrestle with his dock constantly switching from one to the other with no way to lock the dock and keep sane window setups.
I’ve never had this issue? What is your setup? I always thought it was funny that in a Mac you can’t set your default monitor, but I’ve never run into the issue you are describing.
What you term "alien, obscure, and mind-bogglingly bizarre" is the only way I think should work.
Shouldn't you, though? Display arrangement lets you relocate the menu bar—imo that's as good a signal as any about which monitor should be the default.