If an AFCS malfunction lead to a ditching I would be staggered. The S76 can fly perfectly well with both AP turned off, I've even seen some guys fly an ILS in IMC with both AP off (in the simulator).
The ECL says turn off the faulty channel. If by some remarkably bad luck there is a fault in both channels then turn them both off (or perhaps accept the least bad one engaged). If there is binding in the controls with both AP off then you do have a situation where ditching might be a course of action (the binding would be unrelated to anything AFCS).
Ditching with both AP off would be a tricky manoeuvre, you'd want an AP on for this even if it was bad in one channel.
There must be more to it than this. Hopefully something useful has been captured on the FDR because bench testing all the AFCS components/sensors would probably be pointless after salt water immersion.