To clear the name change and building societies out of the way first before we conclude with the forms.
1.The name change and the mortgage are two entirely separate things, there is no link between them. You can change at any time. There is no liability on the part of your wife. The only thing is that whilst the joint tenancy exists if you are called to the hereafter the property automatically goes to your wife (and vice versa) and contrary wills, challenges etc cannot alter this.
2 The Halifax "legals" are childish simplicity - once the �65000 has been cleared to them they send a very simple form to the LR removing their name (the charge) from your Certificate. That is all, no forms for you or your wife or anything. Nationwide should co-ordinate this - see below.
3. If you wish to take advantage of the Nationwide deal asap just change in joint names. The LR do name changes now in days rather than the previous weeks, but if you hit a snag whilst they will guide you through it can revert to weeks. If you change in joint names then your wife will have to sign the Nationwide forms.
4.The Nationwide legal fees should include everything except the name change but including receiving your �15,000 and combining with their �50,000 so that only one final cheque is sent to Halifax. Without going into the rigmarole two cheques can cause a problem.
5. Nationwide should also,importantly, co-ordinate the removal of the Halifax charge from your Certificate and and the submission of the LR Form for the entry of their own.
It is important that they do this,