what happens if based on UG you therefore bring 3 bags at 70lbs but then the UG gets taken away due to aircraft change or crew? How should one negotiate this with the GA?

I would expect CO to not charge, but in some cases there are luggage embargoes where you cant even pay to bring the 3rd bag. Admittedly this should be a very rare occurrence, but since I travel to Mexico regularly it can easily be a problem (have already had to argue with GA's about baggage limits before). If a GA refuses to exercise some discretion and allow the bag(s) they must be left at the terminal.....