Making it as simple as possible and with all the ideas over the years .... if Sonos could do this I think there would be a lot of happy people and it would be a good selling point.
Call it Jukebox Mode - put a marketing spin on it.
In the More ... menu put an option "Jukebox Mode Enable/Disable". Make it very easy for someone to see it is enable or not and this would be best place vs. an icon in the queue. And not buried in settings menu.
When someone enables you can put a pop up box showing what the option does.
When you enable Jukebox mode ....
On all the selecting music screens clicking on either the song name or the ... button does the same thing. Brings up menu option that only gives option to Play Next or Add to End of Queue. Those are the only options people are given.
That would satisify 90% of needs when it comes to a party mode. Don't worry with disabling rooms, disabling volume control etc. Those aren't big deals. The big deals are people selecting play now, replace queue, one clicking (which is play from here) etc. Those options would be locked out.
When the menu with Play Next or Add to End of Queue come up it can spell out - this system is locked in Party Mode options are as follows. Have it look like the same pop up that comes out now that says queue has been edited. Except now it gives you Play Next, Add to End of Queue, Cancel. That's it. If you wanted to go real cool - where song normally pops up saying it has been added to queue. In party mode have that screen say "Your selection has been added to the Queue - your request is currently Song # ?" That would be cool.
People who would mess up queue normally aren't going to know where to go to actually look at and re-arrange queue. I can always go into the queue itself and re-arrange peoples requests at my leisure. Nothing else is locked - party goers could do that to if they wanted to and knew how its not a big deal.
This simple locking would be a terrific benefit to many people.