Surely we have exhausted most reasonable causes and are now going in circles - it must be time now to wait for the NTSB to give the answers.
I did not find any discussions around the initial sounds, that was the initial events that we can distinguish.
If the Blancolirio YouTube video (posted earlier) is correct with sounds and timing, the helicopter was flying approx straight and level when these ~1/rev frequency pulsed noice arose.
If the sound matching is correct, I’m having a hard time seeing it is not a clear lead to the cause of the accident.