I have seen no end of posts about Annulment, but all the ones I remember have been around getting the non USC deported.
I am pretty sure you are the first one I have come across the other way around. Well there have been 2 cases where a USC was able to get an annulment, what happened to the ex Spouse no idea.
It is nice to have a real world example of a case, for the next time it comes up. I know you can remove conditions with an annulment but it can be very difficult to persuade some people that is the case.
Unless the system has an Annulment option I would go with the officer - Divorce.
Caveat I do not see everything.