I pretty much never want to have to work on the wipers on the side of the road in a downpour again. Pictured here are the parts of my fix, and examples of the original linkage in various states of disassembly. The hole for the pin was a lot smaller than I had expected, and I'd already bought the hardware, so much of the following modifications are in fact unnecessary (I simply didn't feel like another trip to the hardware store). If I have to do this again, I'll tear down the linkage prior to buying hardware. Smaller gauge stuff is cheaper.
After learning the shaft was smaller, I did away with the circular nibbling of the mushroomed rivet.
Best approach with the motor end of the linkage was to cut the stud off with a hacksaw. In retrospect, that would have been a quicker way all around.
I had to drill out the linkage arms as well, in order to accept the collar of the plastic grommet. This was a messy business as my clamping capabilities were not up to the task of the bent rods and the drillpress I was using is essentially cheap junk and has terrible radial play. I think I have a line on a superior tool, which will allow me to tear this thing apart and see what I can do. But I digress, no pictures were taken of this as the quality of work was substandard. It'll carry the load set out for it, but it's not worth advertising.
The connection on the left was a bastard to break free, since I was still in the mindset of dealing with the mushroom end of the stud. Anyone recreating this work is HIGHLY advised to simply cut the original stud off with a hacksaw. Unless you really feel the urge to punish yourself, then you can work the mushroom down with a punch and pliers like I did. (seriously, just cut it off)
Not bad. A bit of Loctite on that nut and it will be ready to go. I should mention that the plastic bearing collar was cut down, it still sits proud a bit, to prohibit the two arms from touching, but the original dimension would have built in a lot of slop (and minimized available thread for the nut).
I also tore down the motor housing and transfer box, cleaned and lubricated everything and now it's just a couple bolts (aluminium, one sheared at teardown) and it'll be ready for new blades, which are sure to be a real PITA.