Roadside bodging (first and second gears not selecting)

[ChristoT]

Even if I did ride with my foot on the lever, which I presume nobody does since it's likely to result in unintended gear changes,

I agree, but you'd be surprised.

I don't see that it would cause undue wear.

If there is light pressure on the shaft, the moment may not be enough to change gear, but it's putting parts under greater stress. Hence the accelerated wear.

As for the second shaft also succumbing within a few thousand miles... I've no idea how many miles it had done prior to my acquiring it nor how brutal its previous owners were.

Granted, however, the fact that it was fine for a few weeks is puzzling. Could you have misaligned anything as you rebuilt the gearbox?

The fact that the mechanism clashes with the pins of the selector drum once the claw has a millimetre or so's free play and becomes an "end stop" long before the proper end stop comes into effect points to a design fault.

Agreed.