Could this be sound of rotors beginning to stall as pilot desperately tried to gain more clearance passing over the ridge? Or am I trying to read too much into the audio?
No and yes respectively to your questions.
The pilot was doing what poorly trained people do - convincing himself if he stayed close to the ground he could make it over the ridge.
As others have said, it is best to approach a ridgeline from a 45 degree rather than head on as it gives you an escape option with an easy turn.
However, my rule #1 still applies - don't cross the ridge unless you can get high enough to see if it is clear on the other side. He didn't and was then surprised by the cloud clinging to the trees on the other side.
Once in that position he had essentially 2 options - A. punch up into IMC and climb to a safe altitude (probably not instrument rated or practised in the skills required) or B, try to turn round and get back over the ridge.
He tried option B, became disorientated and hit the trees - game over.