I think it would be difficult to clench your teeth so hard you would fracture a front tooth. And if you were clenching that hard, your dentist would probably see evidence of bruxism on other teeth besides the one.
I've had a little bit of experience with bruxism and I can usually tell when its been happening. My jaw will be a little sore. Some of my teeth will be sensitive. And sometimes I'll even catch myself when I am half asleep in the act of clenching my jaw. But, I have not had any problem with bruxism since I started PAP therapy a few weeks.
I too use a nasal pillow and it does not cause me to clamp or clench my mouth. My mouth stays shut effortlessly. In fact it requires effort to open my mouth when I have my gear on.
Sorry to hear about your issue. Good luck.