Any biometric would probably be good enough, but to satisfy the second point, it would probably have to be a combination of more. The problems would would really be the opposite, not how to create a unique hash for a person, but how to recreate it, because scanners are not precise enough etc and you'd be locked out. The real problem of course  is 3, there  could be a decentralized system that would recognize your biometric data as that datatype, but you could always (in principle imo) spoof it with similar data. At best you could make a system that would employ many different solutions against spoofing and get a high probability of a unique person. For this person to have a fixed identity would again be impossible to solve with certainty and a whole other set of solutions against swaping/trading accounts would have to be developed.