2. If the letter requires a letter from a professor, it must be from a professor who taught you in a class. Most schools I've seen usually differentiate "character" letters (i.e. from PIs, bosses, volunteer coordinators) from professor letters. If your PI also was your professor, I don't see why they can't write a superb letter talking about class and out of class experiences. Then you can probably submit the letter under whichever category you need!