Re: Can he block the mortgage assumption process?
A call to the mortgage company may be helpful, I am sure this is not the first time they have run into this situation. A call to your attorney may also be in order to run interference while you complete the process, as well as help navigating the assumption with a hostile cosigner.
From a legal point of view, he has no basis to come at you if your divorce decree is final, he did agree to all of this. But the mortgage company is under no obligation to follow your decree, as it is between you and your ex. This is where your attorney may help.