There are over the counter mouth moisturizers that work, not 100% but I find them helpful to reduce dryness of the mouth and throat from breathing through the mouth. They even make toothpaste - suppose using toothpaste, rinse and ointment may add hours to the protection... I don't know I have used one othe rinse and ointment, one at at time.
If I have apnea it hasn't been diagnosed, so I have no idea what a CPAP is or how it helps. I did read your description of it being a mouth/nose mask (with oxygen or some other help?).
I use a lot of saline spray and a prescription nose spray: Fluticasone (generic) but what really works is just over the counter Afrin which can not be used but a few nights in a row. When stuffy it is usually on one side, the low side. If I lay on my left, the left nostril is closed, and if on my right the right nostril is closed. It takes only a couple of minutes for the plug to shift sides.
I do not have pain, and I think that is an indication of an infection. I suggest you see you doctor about the congestion - may be a specialist ENT.