This is not to "disrespect" any of the f/o's by any means, but you are the PIC and the final arbitor of the decision making process or at least should be. We could argue this all day long.....but here could be an abstract case or two. You, the only captain on the a/c. Both f/o's have not been allowed or failed upgrade. Things happen while you are in the CRC....the company says no problem because both f/o's are qualified etc. But then shouldn't they be allowed to upgrade. Or what if they both have only a few hundred hours on EK aircraft. By EK standards they are not qualified to upgrade, but can make all decisions during an emergency with you, the captain, unable to physically make it back to the cockpit.
To me your post shows that you respect the F/Os in your company but not trust them which is actually disrespecting.
In your case you should never take a walk to the toilet, stretch your legs in the cabin, take your meal or close your eyes in the seat and let the FO be a PF on the sector. If you have a bunk just behind the cockpit and are in deep sleep what difference does it make where are you? Maybe the FOs should bring their training records to show the proficiency to the honorable supreme commander in chief of the flight from A to B
Do not try to put this as a justifiable reason for a blocked first class seat.
Never been on the 777/A380 crew rest area, but if it is a health or safety hazard than suing Boeing/Airbus is way more feasible.
"dire emergency"...please...