So the question is why did the crew select it OFF, so soon before crashing? It seems to me highly probable that action was a desperate attempt to resolve a serious malfunction that they suspected, rightly or wrongly, was caused by EMI.
So if I understand correctly the scenario goes something along the lines of:
1. ZD576 approaches waypoint A.
2. Crew select waypoint B.
3. At that moment or very shortly there after some sort of spurious caption/engine run up/UCFM occurs.
4. Crew link 2 to 3 and switch off TANS in attempt to clear 3.
If so seems possible to me.
EG