Wow, tough situation, tough question to be asking. There isn't a book somewhere that has this kind of thing written down. Ownership normally starts with shares, like shares of stock, even in small businesses. There are no formulas.
You should probably be talking about this to an attorney.
Celebrity endorsements work when stars behave. Not when they go off the rails on the crazy train. Now AIG wants to come to the rescue with a way for businesses that dance with the stars to cover their butts.