Your plan will probably not work. The mortgage holder will need to agree to the change in the ownership. They are under no obligation to due so, even if the payments are being made. In fact, your death may cause the entire amount remaining on the mortgage to become due.
Eh? The mortgage holder has no say in ownership changes. However, since the heir is not a relative, the preemption on the due-on-sale clause, it is the case that they can call the loan.
Further, just because you put something in you will doesn't mean it's going to happen. Heirs only receive things after the obligations of the estate are met. It would behoove you to talk to an estate planning expert.
I can't imagine any sort of life estate that involves them continuing to pay you rent while you are alive.
A longer term LEASE may be a better option. Just because you die doesn't invalidate the lease.