I also use the P10 mask. I have the same issue with mouth leaks, so have been trying a chin strap,
which doesn't fit all that well and tends to loosen up, so I put it on first, then put my mask over it,
which seems to help a little. A chin strap is somewhat like a mask, you will have to try different ones to see what works best for you. I also practice holding my tongue to roof of mouth, after awhile it becomes natural.
I don't know much about the appliances, only read some negative stuff that it could in time shift your teeth or natural bite.
You are correct in that the chin strap seems to pull the chin back. I haven't checked into any other chin strap that may work differently, like pulling it forward. I don't know if they actually work that way.
Maby someone else here can shed more light on chin straps. Anyway, a chin strap should help with mouth leaks. Have a good day!