Evidence for the chemical imbalance theory comes from research. Before the ability of electricity to create a seizure was discovered various other methods, including intramuscular and intravenous injections of chemicals known to cause seizures were used. The ability of spontaneously occurring epileptic seizures to relieve certain mental illnesses was noted in the last century and led to a search for a safe and effective way to create seizures in non-epileptics. It is thus the seizure and not the electricity or any other means of causing it that is the effective thing in convulsive therapy. Modern ECT is always done under light anesthesia and after a powerful fast-acting muscle relaxant has been administered that totally blocks the visible bodily response to the seizure.
Hypnosis is simply a means of focusing attention.
There is nothing magical about it, however. Trying to access the unconscious directly by hypnosis is like trying to go someplace without actually taking the journey. The actual effective and lasting part of most therapy involves confronting and working through resistances and repressions, not bypassing them. The journey is actually the destination. There is no category or phenomenon in psychiatry called split personality.