I cannot see why the Nationwide would be concerned about your IVA, if they are a fully secured lender. They will, of course, be concerned about your mortgage arrears, and if you are to try and clear these over the next few months, your IP will need to be informed of the situation as this will have to be funded.
It sounds to me as if the burden of having a mortgage, and an IVA, has become too much - and perhaps one has to go. Which one, will depend upon your priorities, and if you go bankrupt as a result of a failed IVA, you may find that you lose the house in any case.
I think a chat with your IP is in order to see if they can make any suggestions to you.
Regards, Melanie Giles, Insolvency Practitioner