Bos,
I speak from how we operated the SK4 so may be a little different to the CH47.
The HSI was only relevant in IFR flight. It could be used to track a VOR, but only that. In VFR flight, the TANS course would be displayed on the 'Charlie bar' which was the lateral element of the doppler instrument used in the TANS mode, either tactically, giving a course to next waypoint, or the other mode (sorry many years ago, forgotten term) which would return you to the direct track between previous and next waypoint.
I think the HSI could be slaved to the TANS, but it was a clumsy instrument in that mode and never used in that way.
I know if I'd crashed, the HSI course woukld have meant nothing. Similarly, the ADF frequency is irrelevant, they simply weren't using it.
Were I flying the accident aircraft, I'd be using TANS in TAC steer mode and a map. Nothing 'radio' wise would have been in any way used or their course, or frequency settings relevant.
The waypoint change, to me, suggests that the next point had been visually identified, so the next was selected. What happened next we will neber know.