This was the primary conclusion that I came to as well. Quirrell has already demonstrated a propensity for playing multiple identities. Plus, if I were in his place as a professor needing a fallback secret identity, I would want it to be someone who nobody would find me suspiciously incongruous with on account of it actually also being me.

Also, the fact that the unnamed person lived in isolation, and was estranged from his family and friends, is evidence for this hypothesis. If he's under Imperius, being impersonated with polyjuice, or otherwise being magically replaced, Voldemort wouldn't want people around him who would notice any sort of incongruous behavior, or force him to play a role full time.