Thanks so much for all the advice and information, very helpful. I also did a lot of other looking around. To cut to the chase - I've now fixed the issue
So basically I removed the column and did a lot of measuring etc, then just separated the key housing (which is attached to the shaft) and pulled that out as far as I could to take a look. I was able to confirm the measurement from the top of the shaft to the first pin was about 2.5" shorter than expected. So I held the bottom of the shaft in my vice (suitably protected), made a bracket for my slide hammer, and pulled on it. To address an earlier comment, this would cause no damage because the rest of the column is not connected to the shaft. you are literally pulling one end of the shaft from the other. I double confirmed this by separating the top of the housing as mentioned.
I needed some pretty serious pounding to move the shaft. I basically had to bite the bullet! I could see that the shaft had moved as a result but had to go back a couple of times to measure and pull again. Eventually I measured and appeared to be at exactly factory length (a bit of judgement and quite a bit of luck). You can see in the 'after' photo the part of the shaft that had collapsed by looking at the surface rust marks.

With it taking so much force I was not worried about securing it further. If I was I would have used Loctite 660 which was basically the factory 'repair' method.
I took it to the shop finishing my car for me to do a trial fit, confirmed it was spot on, then took it home, finished reassembling, and a couple of days ago took the column in , fitted the coupler, and properly fitted the column.
So I'm happy with the result after a bit of an initial shock, still a mystery how the shaft shortened after I removed it and took a lot of care, but it is what it is. It took longer to research it than to actually fix it, but did not want to take any chances.

By the way, I agree with the discussion about the nylon ring and the 'shear pins' really being injection holes, but the factory does use that exact terminology. Regardless I just don't see the 'pins' doing much of anything, and in all probability had already fallen out on mine long ago. It's the nylon rings providing that (considerable) resistance

So thanks for the advice, I thought I should at least provide a conclusion which might help someone in future!