Knowing most of the events/players of this scenario, it would be most desirable if all of the cards somehow got back to A. However some are too far dispersed at this time and it also requires someone or more than one take a hit, as the person who stole them and sold them to the dealer has no skin in the game.
M has little chance of getting reimbursed from B and B has no chance of getting reimbursed by the culprit, (sure would be easier if we could use names, the initials confuse me)
and A has little chance of ever getting back to where he was in this scenario before the theft.
I think if M does the best he can to reassemble the original group and makes a good faith effort to make A whole, at the very least he makes himself feel better about the entire mess and gives A a large portion of the collection back in it's proper place. At that point it would be nice if B would step up and reimburse M at least some money to help out. No one is going to be completely unscathed by this entire deal, except for the perp who could probably care less.
All that being said, this is probably not the best venue to try this case, as most outsiders will not have the complete story(at least as we know it).