Is Quirrel aware of all the people whom he can possess via his True Horcruxes? Can he possess any one of them at will, without the original body dying?

This may explain the fact that he occasionally leaves his host body. We thought it was to inhabit his other horcruxes, particularly the Voyager one, but it may be to possess other people.

ETA: apparently the answer is yes:

Professor Quirrell smiled slightly. "I had many years earlier considered making [the Resurrection Stone] a horcrux, but decided against it at the time, since I realized that the ring had magic of unknown nature... ah, such ironies does life play upon us. But I digress. You, boy, you brought that about, you freed my spirit to fly where it pleases and seduce the most opportune victim, by being too casual with your secrets."