it actually would work (reasonably close) to how its described in the game, and yes is perfectly legal.
the will left the assets to a trust, the trustee either awards assets or donates them to charity. thats it, thats where the will ends. what is left is a contract with the trust that the trustee and petitioners can follow or not. the trust is financially obligated to adhere to conditions as that is how it nets the most from the contract, if they choose to ignore it the proceeds just go to charity.
the petitioners are free to try and claim assets or not. nobody is being forced, its all legal.