This is not just to do with Mamasans. I say that as my emotional thermometer used to be very similar, trying to be polite and tolerant, but without an internal mechanism to send out readable warning signals as you pass the midway threshold of irritation, until I/you explode. I noted that you also had this issue in other situations.
My coping mechanism now as I get old is to try to recognise that point in myself, and, with a smile, gently say "You are really starting to piss me off" or the appropriate cultural equivalent. I find that just saying this gives my patience a new lease of life, although it's usually not needed as the interlocutors get the message, although sometimes they do a double take first!
In the end it is always about what you and the guy agree privately and then what you both feel comfortable with in the room, and nothing to do with mamasan's agendas. They always have an agenda but sadly it is often not about making both the guy and his customer happy.
How many times do they mislead a customer about a guy and how many times do the customers go back to complain about the deception, not often, understandably,.........but by then the money is in the till or mamasans' back pocket.