this is NOT a leverage

Quite the opposite, they may very well view it as blackmail. What you did was wrong. They let you get off easy by laying you off. If you want to push them, they may go over the edge and send you directly to the court. If the accounting/tax issue is grey at best, and the family is wealthy (say, a Rockerfeller), most likely they HAVE consulted those private weatlh management professionals, and whether you know it or not, have sign offs from those big shot firms. I just don't see how you could possibly expect to get better end of the deal by blackmailing them.