Flying 3 axis coupled at Vy, ALTA capture at MDA and a failure to raise the collective would produce this exact effect and its progression is exponential as the IAS slides backwards up the Power Velocity curve.
Agaricus, the AAIB have full access to the crew. if something broke and caused the reduction of airspeed the AAIB bulletin would have been worded very differently!!
As it stands it implies there was no technical error.
We know the crew personally and you do not. Knowing them as friends and colleagues does not absolve our desire or responsibility to seek the truth
All the while explaining how this couldn't happen in a 225. My friend either you know and have it all worked out and that's cool, but unsure the need for this run around. OR you don't and you have thrown them under a bus. Not sure how else you term it in Scotland.