You have a lot of questions there, but it's possible to tackle them all, albeit briefly. I'll insert brief answers -- recovered from memory -- into your text between asterisks, so please read on..
Were both rudders linked?
***Yes, but they could be worked independently as well.***
Is it an electro hydraulic design?
***Electric motors running a worm and pinion type of arrangement, fairly standard for the period.***
Were the divers going into the steering flat in order to move a coupling?
*** little firm information in surviving narratives***
In terms of blowing the rudder off - I can't see how this could be done with any certainty of not destroying the propeller.
*** A very unlikely process for either rudder in my opinion.***
I have seen some threads where there was talk of removing the rudder stock by undoing the nut and pushing the stock down. Having removed these from 30,000tonne ferries in drydock.....believe me....it's not something you would do in the Atlantic, let alone in a flooded steering flat.
*** Totally agree, and totally useless. Starboard rudder hopelessly distorted and wouldn't move anyway, port rudder gone. ***
Where did the information come from about the starboard steering coupling being disconnected?
*** Don't know. It would have been a good approach to try in any case.***
Similarly the talk about the Port rudder being absent. Surely if the Port rudder was blown off, the engineering staff would have worked that out?
***We don't know how much the crew knew about the damage around the bottom. It's my belief that the port rudder separated from the torpedo explosion, though it may have come off as the ship hit bottom and/or during the slide afterwards. I consider these latter possibilities much less probable. The nature of the break suggests that there would have been little indication inside the ship that the rudder was actually gone.
If it were so, the Starboard rudder would have worked, albeit manually.
*** Not a chance. The starboard rudder was hopelessly wrecked, and wouldn't have turned in any case. The entire mechanism was displaced.***
I realise that any manual operation would have meant low speed manoeuvring. However, once on a heading and in the 'midships' position, speed could be brought back up. Surely the steering flat and emergency con position would not be below the waterline?
On a slightly different tack. The results on some vessels going astern...with everything in working order, are nowhere near as predictable as many people seem to think. The realities of manouevring a very large vessel are somewhat different when it comes to trying to ascertain why the stern flies up into the wind on one occasion and yet...next time runs dead straight.
*** With one rudder gone and the other completely jammed, no steering at all was possible. I concur with your assessments of attempting to steer otherwise, or only with engines.***
I'm sorry if this has been covered....I have read a lot on here, but still not seen any answers.
*** Hopefully this helps a bit ***