The usual issue in these cases is that the lender won't allow a wife to take over a mortgage in her own name because her income does not justify it. Are you sure that this is not the issue? Has the lender confirmed that they are happy to let her take over the mortgage in her own name subject to the life insurance issue?
If so, then just continue the joint life cover. If she can't get the insurance, it's presumably because her health has deteriorated since you took out the joint policy. In which case, it makes sense to keep the joint policy going.
You might have to do an agreement that if you die, your estate will get the proceeds and not her, although they will clear what is left of the mortgage first.
Brendan