Requiring a "supervisor" key pair and a "user" key pair to decrypt multiple-recipient messages
Have you considered using Shamir's Secret Sharing algorithm?  

First, Bob encrypts the message with a symmetric algorithm.  Then, he divides the secret key into four parts using SSA such that it requires three parts to decrypt (a $(3,4)$ threshold).  Bob then distributes one part each to Alice and Mark, and two parts to Jim.  

When Alice wants to decrypt, she needs has to have both of Jim's parts plus her part.  If Mark wants to decrypt, he similarly needs both of Jim's parts plus his part.  

Alice and Mark cannot collude to cut Jim out of the middle, because they have only two parts between them, and decryption requires three.  And because Jim has only two parts himself, he cannot decrypt the message unless Alice or Mark shares their part it with him.

Finally, Jim can withhold his parts from anyone he no longer trusts.