Hot debated topic in just a single day around 50 posts.
I didn't read everything, stopped after post 24 or something.
For me a system would be sufficient that characterizes 3 groups:
Major can of course decide whatever he wants with his beacons / plots and everything inside.
Villagers should have access to everything in the way the major has - except to locks and locked items (and the block the lock is on). Locked plinths should work for villagers as for non-villagers.
Non-villagers can enter every free space, but just interact with plinths.
I guess that I myself would not put villagers into different groups. Either I want someone to interact with my space or not. And if I want to build together but not share my items, i put a lock. Even my dearest friends would not have access to every coin I want to earn and every high level item I achieve.
My solution for locks placed from villager's in Majors beacon: locks of villagers do not count for Majors. Just for other villagers. If the villagers does not like the Major to mess with his stuff... well he has his own beacons. Of course this could create argues between villagers, when one starts to block "interactables" for him and the major. In this case it would be maybe wise, if just the Major can place locks in his own plots.
Locking doors absolutely makes no sense to me by the way if you don't build a room made of diamnt walls.
If a villager wants to get in, he diggs around. Non villagers anyway have no access to closed doors right?