Many thanks again for the suggestions. Posts like this are at the same time very encouraging, because I learn new solutions to apparently insurmountable problems, but also sometimes frustrating because there is so much to learn...
Point taken re: Cold-setting; I remember seeing pictures of someone cold-setting a frame. It looks simple, although I don't know how you can be sure the two sides are "adjusted" to the same distance.
See my comment above about "so much to learn": It's sometimes hard to learn this at work because most of the training is about modern bikes.
The new frames are both 130mm, as they both had a modern 8 or 9 spd freehub cassette in them when they arrived.
I get on very well with my Shimano 8/3 bottom bar shifters, so I'm not keen to change tpo a 6 spd but that's useful information if a customer brings a similar bike.
I seem to ride most of the time in the 11t sprocket on this bike, not something I've experienced before on my heavier longtail. I'll check the chainwheel: the bike is an old MTB, and I just re-mounted the old chainwheel and mechs on the front with a new rear mech. I'm quite sure your disagnosis is correct there. As you say though this is a relatively minor issue.
I doubt you're missing anything, but we may be speaking about two different things. When I talk about a "horizontal dropout" I mean one of these:
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I don't know how well tug nuts would work on this: I have them on my Bakfiets, but that has a frame designed for them to fit. I mean a "short term fix" in the sens that they somewhat defeat the point of QR skewers, and it would still be a bit if a faff to change a tyre on the road. In reality, I use Marathon plus tyres and change them when they finally wear out, but still...