@chris_lz:
I'll leave the technical discussion of procedure to the pilots, but the INS track was not used for the descent below MSA, which was offered by Mac Station under "radar vectors" and taken up under VMC rules by the crew. The INS track was re-engaged once they were at the assigned alititude, but this was probably only done to provide a guideline position as they re-checked their position visually (having checked and cross-checked on the descent to the best of their ability). Unfortunately the sector whiteout combined with their position gave them the impression that they were somewhere other than where they actually were - and the AINS track based on the changed co-ordinates meant that their backup plan (without their knowledge) had also been foiled.