If there had been no GWR, there would be no doubt the property was an asset of the daughter and subject to IHT on her death.
GWR is a deeming provision and I do not see why this should necessarily prevent the property being taxable in the daughter’s estate. Although there are various provisions to prevent a double charge to tax, in some instances I understand a triple charge to tax can arise (and none of them is relieved).
In view of the background, was there any intention to make a gift? It could perhaps be claimed that the only gift to the daughter was the contribution to the mortgage payments, the property having been transferred to her upon bare trust for the mother. However, that could spark an unhelpful discussion with the mortgagee.
Paul Saunders