1) dry mouth and flat top leak rate is pretty much a confirmation signal for mouth breathing going on.
2) If the leak rate is still less than 24Lpm on your Resmed then your therapy pressure is still effective.
3) If you were on 10 fixed, then why go to 7-12 variable. Maybe a 10-12 variable will be better. This may let you go back to using a nasal pillow mask which will help in multiple ways:
a) A lot of people report needing lesser pressure on nasal pillows than on FFMs.
b) may help you get back to a 10 fixed or 11 fixed.
You should ensure that you have a copy of your latest sleep study. they may have titrated you to a higher pressure for some good reason which should be in the study. Experienced members on this forum can assist in evaluating the sleep study.
Resmed is pretty aggressive in raising pressure. Resmed pressure chart typically show cycles of fast rapid climb and a slow progressive decay. Respironics raise the pressure slowly and lowers it slowly. But they have their own quirks (Hunt n peck algorithm).
RDI always below 1. But sleep much much better at straight pressure.
Started on F10, Tried Quattro Air successfully. Finally settled on P10.