Here's the deal with ultimatums; if you give it then you better be prepared to back it up. Don't "use" an ultimatum as a "tool". There is a very strong chance she will not choose you over the OM. So, if you've given an ultimatum and you don't carry it through....what does that prove to her?
Don't throw threats around thinking it will shake her up and she'll find her senses.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!