Many CPAP/BIPAP users find that they are breathing some through their mouth. Chin straps work okay in some people that their mouth just relaxes during sleep and droops. True mouth breathers may need to use a full face mask and may actually be more comfortable. If you are having to tighten a chin strap so tight that you are uncomfortable, I would recommend a full face mask.
As to the salty taste, unless you are using a new medication, My only thought is that the BIPAP with the humidifier may be adding more humidity and increasing sinus drainage.
Debbie Shirley,RRT