So what if captain A elects to press on, and is relieved by captain B because of the flight duty time. Could there be a problem if this captain B does not agree with continuing towards Kuwait?
There is only one legal commander and that is the operating captain. The captain who is part of the augmented crew is a subordinate. Having said that any decent captain would, of course, discuss, listen to and take into account the opinions of all his flight crew before making a decision on how to proceed.
If I remember correctly, BA landed short of LHR (Stansted?) with just 9,000# of fuel.
Manchester actually and they landed there despite having sufficient fuel to make LHR.