I love this kind of issue. I have real world flight experience in the SH-3H Sea King. Both the Mi-8 and SH-3 are similar to a point. In the H-3 we would use one engine for the main drive to power the rotor-head and the other would be backed off to power auxiliary systems. Lately, I have have been flying the Mi-8 in the same manner. I have had no issues backing down No. 1 to idle and using No. 2 for the rotor. I was shocked to see this actually modeled. I just assumed it was not.
Now as for unloading that rotor. Just like the H-3 you have to calculate your descent. Every approach to the USS Saratoga CV-60 (AGE ALERT!!!!) was slow and deliberate. You can do a tail whip and unload a ton of speed but you have to ready to go from forward transition to hover in a few seconds. And, the transition from forward flight to hover is modeled beautifully. I often thought the H-3 was going to come apart even at slow transition. Once we were coming into Patrick AFB, FL, on our way to AUTEC, Andros Is. Bahamas and the transition was so violent we thought we we lost a blade tip. Again, beautifully modeled with the Mi-8. Glad you figured it out!! :)