O come on, now... that one’s easy... He used the singular form of the verb because he was talking to Peter, not the 12.... but he also gave the authority collectively to the 12 a few chapters later. The point is that you can’t give ultimate authority to multiple people, so each bishop (heirs of the apostles) has authority on his own in his own jurisdiction, and they have collective authority as a body to which Peter is first among equals.
Not according to the early church fathers, as I have cited already.