GP said they would make an exception and let me combine.
Agree about multiple reservations.
The whole thing is rather odd. For example, presumably you could make a reservation, transfer points against it, then cancel the reservation - the points don't go back, I don't think.
IIRC, Marriott allows combining only within immediate family. Which would be fine for GP if they want to actually make that rule.
Anyway, at least for now, I have it worked out.