CPAP mask fit. The one that fits most comfortably yet provides the necessary delivery eliminating any significant oxygen desaturation. There are many available so there should be one that is comfortable yet provides a good seal. Many have associated restless leg syndrome or periodic limb movement disorder, therefor aggressively treating that effectively seems to allow for more comfort with cpap.
Varies. Varies. Many parameters to assess: A reference here: best cpap mask.