Thanks, I understand now!
I have succeeded in my replacement of the control arm. I didn't bother with the vertical in the end (though I might yet go back to it particularly if they tell me that they still can't do the wheel alignment for any reason). Two things I discovered:
- as I mentioned, with the six-speed manual, the troublesome horizontal pivot bolt is a non-issue. There's nothing in the way
- I got hold of the workshop manual that scarecrowdjinn had shown a picture from and discovered that the white cross on a black circle background (the one that looks like a philips/crosshead screw) that shows up in the image means "do not reuse this part". So, I'll be putting a new nut on that vertically oriented pivot bolt fairly soon
Other than that, guess what? All the experience I got on the left side will soon get practiced on the right side. I took it in for alignment after finishing the left side, and they told me the ball joint on the right side is shot too (just nowhere near as badly/obviously as the one on the left, for which the arm itself was also bent about 20 degrees!)
Anyway, many thanks again for all the help, encouragement, and explanations!