You should be able to sleep in any position that allows you to sleep the best.
If by chance that happens to be on your back and your OSA is markedly worse or need more pressure then sometimes it's just better to let the machine deal with it. If leaks are less of a problem on your back then so be it.
I don't know how much of those flow limitations are nasal or otherwise but you might try a higher minimum pressure just to see if it helps or not. The rather large fluctuations in pressure can make it difficult to keep a good seal. You pretty much have to fit for the higher pressure when you are first starting the night though the above graph doesn't necessarily point to the large leaks being totally related to higher pressures.
Large leak territory on the Icons is 60 L/min...above that is considered large leak.
Finding the right mask is the hardest part to this therapy IMHO. You have to use a full face mask due to nasal issues and full face masks simply have more area touching the skin that needs to seal.
Mask liners help a lot of people so that's something you might look at. There are ready made mask liners (Remzzs or padacheek.com has some really nice ones that last a long time) or you can even make your own out of an old T shirt.
Full face masks aren't as forgiving when it comes to incorrect size...so you might measure your face to make sure the size you are using is correct. If you go to cpap.com and each full face mask usually has a sizing guide where you can print off a template to check measurements.
Full face masks are expensive. Cpap.com has a return insurance policy where you pay a small additional fee and if the mask doesn't work you can get your money back. They even provide a prepaid shipping label.
There are a couple of masks that have free return insurance..so you can save some money there.
This mask is relatively new but I know some people who have had good success with it.
https://www.cpap.com/productpage/respir ... -mask.html
It has free return insurance. Don't let the price scare you off...call for pricing but I think it is $140.
This mask also has free return insurance
https://www.cpap.com/productpage/resmed ... dgear.html
Check cpap.com mask product page as most will have fitting videos available for the various masks.
I may have to RISE but I refuse to SHINE.