There is no reason for the EEC reverting to manual when you loose the Essential buses. They are powered thanks to an engine driven alternator, the so called PMA (Permanent Magnetic Alternator)
The engine high site, has been probably induced by the collective control issue, as some reports mention the pilot trying to control it with no result moving the collective from min to max with no reaction.
As he still had the full cyclic control, the failure were on the linkages between the collective grip and the mixing unit (torque tubes, push pull rod or bellcranks?)