The dynamics of a helicopter with no rotor speed can probably be investigated accurately with a small-scale model that has an accurate weight distribution, certainly to the point of the onset of a "tumble".
Yes, I'm sure you're right. But as already said, no-one has bothered to do it (and why would anyone.....?) so we don't know for sure; although we could argue one way or the other all night long. I'm not going to do so because it's pointless.
However, the failed fenestron theory does also fit in with the evidence of the witness, at least to some extent. A rotating aircraft could be described as "tumbling" whichever way it was rotating through the air. But it's
all supposition, hence me mentioning it because two stopped engines wouldn't normally result in stopped rotors (despite the suppositions of some who have obviously never flown a twin engined helicopter in their life and are talking through their backside).