Where are you getting the money to buy out the joint owner, is the mortgage being increased? If so, then I doubt you have a hope of retaining tracker. If the mortgage is remaining exactly the same then you can ask if they would just remove the name, they still don't have to facilitate you but it is technically possible.
I am sure there is someone somewhere who had done it but not necessarily on this forum, have you put same query on some of the parenting forums maybe, separation/divorce sections, they often have similar type scenarios.