I can't answer your question about PH and OSA. I have COPD and know that COPD can cause secondary PH so it wouldn't surprise me if long-term UNTREATED OSA couldn't also be a contributing factor to secondary PH.
However, if you are sleeping well w/a full face mask it doesn't matter if you mouth breathe or nose breathe. That is the idea behind using a full face mask. I have to use a full face mask because I need a high emough pressure that some air manages to escape thru my lips in a manner I call "the lip flutters". Not the same as actually mouth breathing.
ResScan 3.10 - Resmed S8 ResLink & oximeter
ConTec CMS-50D+ oximeter - Philips EverFlo 5L Oxygen Concentrator
PR SystemOne BPAP Auto w/Bi-Flex & Humidifier - EncorePro 22.214.171.124