If you have rnav in the flight plan the ATC can give you direct to RNAV waypoints at any time, even below MSA. You still are responsible for terrain separation, though. RNAV is not prohibited anywhere.
You cannot fly a procedure for which you don't have a cerfification. If you are not PRNAV certified but you are BRNAV certified and the airport doesn't have BRNAV departures anymore, then you need a conventional SID or an alternative clearance, such as "turn left direct to BUZAD" or radar vectors.
You can fly RNAV all the way from one airport to another without being PRNAV certified, as long as you are never cleared for a PRNAV procedure that you are not certified for.
In your BUZAD 3x, after BUZAD you can follow any enroute clearance they give you. There is no need to lay this fact down in a regulation. The regulation specifies what you cannot do rather than what you can.