I guess telling her that you believe this is a mistake, you love her, you want to protect the kids from the hurt of a divorce and willing to do whatever it takes to make it work. I am talking the day before or day of final papers getting signed.. I guess if we are at that point I know what the answer would be.
So what is different from this and those letters you were sending? I promise you this type of thing is not successful in a stitch such as yours.
As for her softening, don't take it as a sign. It usually happens when the pursuits and fighting has stopped and she feels a little relief from your pressure.
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!