[The ailerons] will not be deflected only in an aircraft with neutral stability, whereas the vast majority of planes (with the possible exception of fighters and unlimited aerobatic acft) are designed to have a fairly substantial reserve of positive stability.
The stability that you describe is lateral stability originating from roll-slip coupling. A slip causes a rolling moment. There is no stability in roll without slip -- the aeroplane has no inherent way of knowing which way up it is, so it can't provide a rolling moment in response to an angle of bank.
Spiral dives exist because the pitch effect is markedly stronger than the roll stability.
Presumably you'd agree that the bank angle increases in a spiral dive. How does "the pitch effect" cause the aircraft to bank into the turn?
In a turn without slip, the yaw-roll coupling causes an into-turn rolling moment. In essence, the outer wing is moving faster and generates more lift than the inner wing.