That is a HUGE no-no. Those performers could have gotten reprimanded or even fired for that. I am a former entertainer, and we are in no way shape or form allowed to be there for a proposal. I was working Liberty Tree Tavern on Valentine's Day when a man proposed. The characters chould not be anywhere nearby. We were appraised of the situation and afterwards went up for a photo shoot and to offer our congratulations, but we could not be anywhere near when it actually happened in case the girl said no.
The other thing is that that is out of character. Minnie always is and always will be Mickey's girl. While Mickey may flirt and joke around from time to time, that kind of behavior is not in his nature.
Please do not risk the jobs of the CMs for your proposal. I'm going to give the CMs the benefit of the doubt and say they may not have known the rule. I didn't learn it until the aforementioned LTT proposal, so it is possible. However, there is a very strong likelihood that they at least got talked to about it afterwards.
If you want characters as part of your proposal, arrange to be at a restaurant or somewhere nearby so that you can take picture with them after, but realize that they cannot be there during the proposal itself.