One of the rear headrest is stuck in it bottom position and no matter how hard I press the release button, it refuses to budge.

Is there perhaps a trick I could use?? Or will I have to open up the seat to get it unlocked?

Silicon spray works wonders on most things... I would give that a go before you try anything drastic  BETTER looking half of the famous Twinlanders
The wanna be other half - http://www.freel2.com/forum/member-carriec.html

fill seat back with just the right amount of propane, light and headrest will pop up as if by magic  At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

Along with the roof of the car  21MY Defender 90 S 3.0 D200

the right amount, not that much  At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

Silicone spray sounds like a logical step which I missed somehow

Failing this, exactly how much propane will be required and does it have to be LR approved?

does not need to be land rover approved but I can sell you a $300 land rover sticker to put on the can if you want to feel better.

or I find sometimes you need to get in the back and pull straight up as evenly as you can with both hands

Just a thought and there is no logic but have you tried moving the back of the seat while trying to release the catch ?