Ob Fwit ...
First off, deeply grateful for your explaining to me some of the nuances of the auto systems.
It seems, from the report, that the crew had not "selected" an airspeed, and were operating with just VS and Localiser coupled. In this instance the vertical mode is being controlled by cyclic, snd airspeed is being controlled manually by collective.
You will I hope forgive me for offering the opinion that this is counterintuitive to helicopter flying. Maybe it is due to me having flown and instructed in both fixed and rotary wing, but I have found that it is best to pick an airspeed as an approach speed, and adjust rate of descent with power (and then of course modest counter corrections with stick/cyclic) when trying to stay on a glide path. I do realize that once one has the landing spot in sight, the transition through translational lift and to touchdown requires adjustments.
I got into a discussion on the Asiana thread about pitch and power equalling performance over at R & N -- I had airline pilots inform me that they were pointing their noses and relying on power via automation to get their airspeed right. That is counterintuitive to how the coupling between pitch and power gats you performance, in a general sense. It took a while for me to understand that they were discussing flying in a mixed mode of hand and auto. While I think I better understand the issue now, I do not find the practice to make sense ... and the Asiana crash is a data point firmly in my favor.
Having taught a bit of instrument flying in rotary wing, a while back, I hold that the same basics apply to flying a helicopter on instruments in actual IFR as they do in fixed wing. Power in this case is collective, as you note.
I do understand the logic behind choosing a RoD in an effort to get a nice constant descent angle. Not a bad thing by itself. Your explanation of the mixed mode traps is an eye opener for me.
If the collective is set too low, ie not producing enough 'power', then the aircraft will reduce airspeed to try and maintain the desired VS.
That's all well and good until one gets below max conserve airspeed, and/or into translation lift since one will begin feel the effects of being on the wrong side of the power curve.
While the details of "the power curve" are not identical between fixed wing and rotary wing, the result of "the bottom drops out from under you" is very similar. I've experienced it in both kinds, without bending metal, and feel comfortable making such comparisons from experience.
However, once the speed passes Vy (which is usually around 68/70kts) the rate of speed drop can be very quick, until at an airspeed none of us can remember the autopilot gives up the ghost and the upper modes drop out.
"You've got it, I'm outta here" says HAL, eh?
In a 225, there are protections built in such that, should for some reason you elect to fly it 3-axis, at 60(?) kts the IAS upper mode automatically kicks in and the ac becomes 4 axis coupled.
It appears to me that maybe those "protections" are overrated.
To sum up:
Please don't beat me up too much for being a dinosaur. I am a bit disturbed with the traps that mixed modes provide when combined with a likely demand from management to use the auto features to the greatest extent possible. No doubt each company will have differences in SOP.
I guess even helicopter pilots must now ask themselves more frequently:
Are you flying the aircraft, or is the aircraft flying you?
Awaiting the more detailed findings with great interest.