I doubt it's from underneath as it needs to accessible for assembly. Just a hunch.
The clutch seems to be two serrated plates pressed together for engaged. They should come apart to dis-engage.
I wonder if that has anything to do with the bolt you found?
Hmmm - if one grub can come undone from a gear, it's possible another one could, from a gear which is not accessible with the 'box assembled. Short of dismantling the whole headstock gearbox, I'm not sure how to verify this.
As for the clutch; the serrated plates were my first idea, but I think that's a safety device in case the spindle locks up. The set of disks to the left of the serrated disks are, as tinkerer says, a clutch pack.
According to the pictures, the clutch pack is staying compressed. If it was stored engaged, they may be stuck together. You may try to gently pry them apart. I had a corvette clutch seize to the flywheel after setting for a couple of months. It was a bear to seperate.
I did wonder about that; however, in the higher gears in particular, the clutch appears to operate properly - albeit not terribly efficiently. I suspect that it requires an adjustment to be further disengaged, but I'm not sure how to do that. I will have to stump up the £45 to buy the manual, I think.