Yep, the nut turned a good amount, but it seems to be stuck right towards the end of the bolt. Wouldn't budge when twisting with my hand.

The 14mm socket worked well for every other bolt, but it's been awkward on this, so I'll try with 13mm.

Good news is the propshaft is off - I can now move the car in drive.

What is still left is to test the VC to determine if it's seized. I tried a bunch of variations (rear wheel up, handbrake off etc), so I can't remember what the setup exactly was, but I was able to move the VC, by hand, ever so slightly before it stopped - as if it now required torque for additional movement.

I was able to move the car back and forth with no grinding noise at all - could this be any indication that the VC is 'ok'?

I'm going to attempt the bolt once again as my propshaft arrives today. Doesn't help that I lost one nut somewhere next to the car.

The nut, or the thread on the bolt has probably been damaged when the broken UJ flew out from under the car. Last resort would be to use an angle grinder and cut it off but that would then mean you will have to take the output flange off the transfer case to replace the bolt.

The viscous coupling is a sealed unit filled with a silicone gel so you won't get any noise from it. Driving with one propshaft missing is a sure fire way of causing it to seize even if it wasn't already (which it almost certainly was). It connects the front and rear propshafts so you need to stop the rear from moving, either with the handbrake or with the gearbox in Park and the rear wheels on the ground. Then you try to move the front, usually by lifting one front wheel and seeing how much torque and over how long, you can move it. But that does assume you have a front propshaft connected. As you don't, you will need to do it with a socket and breaker bar on the nut in the centre of the flange the propshaft fits to.