Mark. Sorry you have this. You have had quite enough woe lately.
However I think you have the wiper issue nearly fixed so take heart !
As I recall, the tough bit was getting the thing dismantled and you've done that.
I've quickly looked through your posts and from what I've noticed, it looks very much like you need the same Halfords clip that I used (Though you will probably have to file it to fit properly. Depending on the clearances between the moving ball joint and the things it moves past you might get away with little or no filing.)
You report that the mechanism worked ok when you popped it back together but that when loaded up with the wipers themselves it popped out again. The Halfords clip is designed to hold the two parts of the ball joint together, preventing them from popping out under load. Works fine on mine which had exactly the same popping out scenario, though possibly for a different ball joint. There are plenty of ball joints in the mechanism aren't there?
So I think you are nearly there. Carry on !
It's my guess that these ball joints are used in the wiper mechanisms of very many makes and models of car. The problem we face is caused by our particularly long wiper blades and the correspondingly large load they impose on the ball joint.
By the way, I threw away the rubbery black plastic sleeve - didn't even try with gaffer tape.
Also, my wipers worked fine in the recent downpour. No reason to think yours will be any different to mine with the same Halfords clip in place, saving you £350 for the new/used part.