I understand your concern here and frustration regarding W's meds, but this is NOT the right approach.
It infantalizes your W and is rather appallingly controlling. I can't even imagine being with a man who would try to get me to make such an agreement. Be a father to your D, not your W.
OT, about anything else, I would agree with you. However, one of the features of bipolar mania is that she may not recognize that she is sick, even when it's perfectly clear to others. By definition, she cannot control her own mind at that point. It's not infantalizing her at all -- that's just the nature of the disease. Her doctor doesn't see her every day the way I do. I agree with my W that she is sometimes over-medicated and doesn't always need to take as much medicine as has been prescribed. On the other hand, W can't control that and "self-medicate" because she is sometimes unable to see or feel when she is getting sick. Putting me in control of her medicine is really the only practical solution.
The affair started when she was manic and wouldn't admit it or take her medicine. Mania is frequently accompanied by a "god complex" where the manic person feels that they can do anything they please, and it is also associated with sexual experimentation and a heightened sexual drive. I do not believe that this affair of hers would have happened if she hadn't been manic at the time. Also, my concern about her refusal to take her medicine while she was getting sick caused me to be frustrated and push her away emotionally, which made the OM seem much more attractive.
Getting this issue fixed is THE big deal for me in any reconciliation.