I also have a small face and cannot use a full face mask, though I have tried several. My machine is titrated to 20 with a pressure relief of 3.
If you are using a nasal mask, try using a chin strap in conjunction with taping your mouth with a strip of medical tape (Nexcare Durable cloth). You can find it at Walgreens and sometimes at Walmart. Some people use paper tape, but I could never get that to stay on. Make sure you fold the ends over about an 1/8 of an inch so it's easy to remove in the morning.
To help with the chipmunk cheeks, you may want to cut down a toenail separator (Walgreens) and place a piece under each of the mask straps and against your cheeks. I put a small make up round underneath each piece of the separator so they don't leave marks on my face. This will put slight pressure on your cheeks so that they will not puff out. Finally, make sure you use the ramp (at least 10 minutes) on your machine. The chipmunk cheeks will be even worse if you are not using the ramp.
I have had the same difficulties you describe and am finding that over time, my brain/body is learning what to do to prevent all that air from coming into my mouth, but it has taken a few months.
Xylamelts (CVS Pharmacy) are also very helpful with dry mouth. You can even put one on either side of your mouth between your cheek and gum line. It dissolves slowly as you sleep.
Hope that helps, and good luck.