Why they cruised at 200ft for such an extended period of time will probably remain a mystery, though.
No mystery. If the cloud base was around 300'.
One scenario is that they "let down" over the ocean. Got clear of cloud. Then followed a route (clear of cloud, but in the dark?) at a height they thought they were safe at.
Any mystery in my mind is why they thought the route was safe at 200'.
The tragedy in my mind is all the safety back stops not working, especially EGWPS.