Just to play devil's advocate here for a moment, Was this extension thing always on the table or has it been introduced recently?
I for one would be happy to leave it to the commanders discretion, with a clause that his decision is FINAL AND CANNOT EVER BE QUESTIONED UNDER ANY CIRCUMSTANCES !! Let's get real sometimes it's better to extend an hour or so than have to nightstop or take min rest. If any of the crew are tired there is no extension . Period!! It has worked reasonably well for the last few years.
However if the Company is using this to blame us for not concluding an agreement, then they deserve everything that is coming their way