Does a durable power of attorney supersede a last will and testament?
Can a last will and testament ever become valid when there is a durable POA?
It is unclear that you understand the difference here in the two documents. A power of attorney is an instrument authorizing another to act as one's agent or attorney. The agent is attorney in fact and his power is revoked on the death of the principal by operation of law. In other words, it dies when the person for whom it is invoked dies. The agent in fact CAN NOT do anything or execute or bind the estate of the decedent once the decedent dies.
A last will and testament only becomes a viable document when the decedent dies. It is the written wishes of the decedent as to the distribution of his or her estate upon his death. Once proven in Court to be valid it is the only viable document that controls.