Yes, an official has the authority to tell a team that he is only going to talk to one of the designated captains or only the head coach. In my coaching days, I had a number of officials tell me and my other coaches that they would only be speaking with the head coach. This was their way to keep the amount of grief they heard from the benches to a minimum.
To answer your first question, a ref has NO obligation to explain a call to a player. I've had coaches get on me because "I didn't explain the call to the player". My response back to him was "that is called coaching and that is what you are here to do, I'm just here to call the game". I will always clearly say what the call was, but I'm not going to have a 5 minute conversation rehashing the play and why I made the call. I'll also only discuss calls if the coach is doing it in a respectful way. If they are screaming at me or my partner, I just ignore them and tell them that we will have conversation when they can do it at a "normal volume".