It seems to me that the one designed to be the main was the hydraulic one but I am a bit confused. For what I have understood there were two hydraulic pumps, one for each engine, drive directly. So why do you say that only one could be drive at any given time?
Yes, it seems you are correct. No timeline regarding the last engine going out of service. Anyway I seem to recall that when the last electric pumps stooped to work (short circuit) no ship engine was working, except one of the generators.
I find odd that at this time of the inquiry the time table of events is not yet completely clear.
Well, it looks like electric were primary.
Hydraulic were secondary.
The problem with them is the way the designer/surveyor friend of the captain installed them.
One was hardwired to the 2" pipe ( instead of 4") and the other to 3" flexible hose instead of 4".
Also they both were driven by hydraulic fluid pump on 1 engine. This pump was 9gpm which was enough to drive just 1 pump at a time. There was no way to run both at the same time.
For the timeline - each crew member gave estimates of the time, but at the end of each day 1st mate was allowed to "question" them and he asked questions like "Could it be that you stood 8-12 watch instead of 16-20?" You see what he was doing it was very transparent..