Robin
I have no knowledge of the EC135, but I am familiar with other very similar (if older!) helicopters.
Each engine would normally be equipped with a starter/generator, typically 28VDC. this may or may not be connected to the engine's accessory gearbox.
Since each engine will always have a freewheel assembly, preventing any engine malfunction 'dragging down' Main Rotor RPM. The main gearbox is, in all cases effectively isolated from the engines (and generators) during true autorotation.
So your theory doesn't hold water! Though I'm not sure the people who know the specific EC135 answer will bother to answer your question.
As I have stated earlier, an autorotative landing is a visual manoeuvre. During the final flare and landing, frankly there wouldn't be time for Rad Alt information to be effectively used. Yes a landing light would be useful but that might not be provided from the battery bus alone???
It's EXTREMELY difficult to imagine a final flare to be so badly mishandled, even in low ambient light in a city centre, that the MR RPM, is completely spent! IMVHO that cannot be the only explanation for virtually zero RRPM!
I know of two twin helicopters that have run out of fuel whilst airborne, and two very large jet liners. In all cases everyone walked away! Such a scenario shouldn't have come as such a surprise to this talented pilot that he was not able to have a pretty good stab at an autorotative landing.
As I keep saying, AIMVHO of course!