Take it from me. It can't happen. No such thing as perpetual motion. No such thing as a direct coupled heat exchanger producing a greater than 1.0 efficiency. Period. End. Done.
Many marvelous and wondrous things are yet to be developed by the minds and hands of man. Just not a microwave water heater with an energy coupling efficiency which exceeds unity.
There is a fifth dimension, beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity.