(1) By a writing signed by all parties, the parties may designate as agent of all parties on an account a person other than a party.
(2) Unless the terms of an agency designation provide that the authority of the agent terminates on disability or incapacity of a party, the agent's authority survives disability and incapacity. The agent may act for a disabled or incapacitated party until the authority of the agent is terminated.
(3) Death of the sole party or last-surviving party terminates the authority of an agent.