We are talking about Marriott Bonvoy, and the rules for award stays have been detailed in this thread: no third person charges for award bookings (as long as the room type allows for the third persons). Yet you say that in Japan there will be problems, and you double down on that. I'm even more confused as before. What are you seeing that I don't?
The thread moved on from reward only bookings to general "third person room" bookings by my initial interpretation. Re-reading your specific post, that was a wrong interpretation for your specific ask, and my apologies.
So if you are saying:
I book a hotel in Japan for 3 people using points; My reservation says "0 JPY", BUT the room rate details say "Extra person will be charged XX JPY" (which clearly contradicts the T&Cs quoted earlier), I think you would be better to raise proactively in advance with the Lurkers to avoid the potential "conflict" where I think (to my original point) a local staff member may prefer to interpret their "rate details" (that rarely correctly flows through on cash bookings from my own experiences) over a guest quoting T&Cs and causing disruption. This may be unfair if the hotel is used to this scenario of course.