Yikes, that does look like a problem waiting to happen on that slave rod linkage.
The US diesel cars we got here (only available thru 1985 with manual trans) all had the old style cable operated clutches so most of us US-based members here may not have a lot of useful info to say about the hydraulic setup yours has. Maybe try sending a PM to some UK members like ukvolvo and see if they have further firsthand experience?
That said, based on what was done to that clutch fork I am guessing there had been an issue of the slave pushrod not being long enough to meet the clutch fork hence their adding that bodge to try to make up the distance. I would think that could be because there might be air in the hydraulic system resulting in insufficient travel of the slave pushrod, or could be a mismatch of parts (wrong master, wrong slave, ???). Those would be things accessible from the outside to check out. Perhaps starting with a very thorough bleed of the system and fresh fluid, / inspection of the fluid that comes out to see if it is dark suggesting failure of the seals etc.
Or maybe a bent clutch fork or damaged fork pivot could cause it. Or the wrong fork, like for a gasser car? Checking those out of course would mean dropping the trans unfortunately so maybe those would be second down the list. I am pretty sure the release bearings would all be the same since from the input shaft back, the transmissions are identical gas and diesel. I doubt that would cause a judder anyway.
Judder on engagement -- I would expect that only to be caused by warpage in the surface of the flywheel/pressure plate, and/or by worn engine and transmission mounts. Mounts are a common wear item on these old Volvos due to the safety breakaway design. Putting in a fresh set of all 3 mounts could solve the judder problem, they have been known to be a culprit for this. Once the mounts are tired enough to allow the engine to jump around during clutch engagement it can set up an oscillation you feel as a judder.
That of course wouldn't solve the issue of your slave cylinder pushrod though.
If I had to guess I would say it's two separate problems, one being the judder on engagement and the other being that clutch release linkage. BUT a cracked release fork or something like this that made clutch engagement inconsistent could potentially link the two!
We will be curious to hear what you find out.