Well talking about the legal ramifications of all this is part of the problem.
For the most part aviation is a very pragmatic industry and that to be honest is the building blocks of modern CRM. Crew Resource Management if not in the know.
When you are in the air and an abnormal occurrence happens such as a capt. incapacitation a pragmatic look at it is needed, as said before at 400 mile an hour we don't have time to play lawyers. A simple unconscious incap is easy to deal with no questions there. An insidious or even worst an aggressive incap is harder to deal with.
As for legal responsibility that where it could get difficult but to be honest only if something goes wrong and you need to prove it was either the correct or wrong decision. Frankly its sad that you would even have to consider this as I can pretty much guarantee any aviator would only ever do what they thought was the safest and most appropriate action. (its our lives as well remember) I honest don't believe any company would have the slightest problem with you if you said I'm sorry but in my honest opinion the capt was unable to perform their duty in an acceptable manner.
Every FO flying should be perfectly capable of performing as a capt. and company training should ensure this and therefore place the trust in their decision making ability just as they do in the day to day operation of a metal tube with unknown numbers of passengers onboard.
Communication and the ability to be humble are very important qualities on the flight deck.
Hope this made sense but i can't be bothered reading it back.