There are a particular type of mirror will
called a mutual will - which expresses the wish that it is not revoked without consent of the other.
reviewed 2011 COnveyancer Property Lawyer
where the conclusion is:
As most textbook writers have pointed out, mutual wills do not come before the courts with any frequency but, when they do, they almost always raise issues of convoluted law for the academic lawyer and significant financial consequences for the practitioner and his client. For all these reasons, there is a need to be clear as to the differences between the “everyday” mirror will and the mutual will.
Still doesnt answer whether the surviving spouse can mortgage some of the property (presumably for maintenance)
and I still think the answer is yes so long as she is give the power in the dead persons will