Boards are pass fail. Pass it the first time.
Residencies in general value GPA, class rank, 1st time boards pass in order to extern at the program.
No, you are not allowed to sign up for the USMLE as a DPM student.
What do you hope to get from signing up for the USMLE as a DPM student?
Being a DPM will not fulfill your dream of being a DO. Your specialty is locked in from day 1.
I suggest you think long and hard before shotgunning back up plans.