Theo

When the valve looks good closer inspection can reveal that it is slightly arched lengthwise leaving a small gap under the centre of the valve. This can be enough to produce the noise you describe.

Concertina Connection https://www.concertinaconnection.com/valves.htm

Not a good idea.

It's not difficult to make a spring. You just need some piano wire of the same gauge.

20 button Lachenals vary greatly in playability. Some are a delight, some are hard work. If you decide on a Lachenal it’s very important to be able to try the instrument before you buy, or at least buy from someone who will allow a return. if you want to leave open the possibility of playing in D you might be able to find a nice 22 key Lachenal. These are not common, but not super scarce.

You don't need acetylene, a propane torch will give you the temperature you need for hardening. Its not so easy getting the right temperature for tempering, especially for small items with complex shapes.

There is another possible explanation for the enharmonic notes on an EC. If they were not there there would be gaps in the layout. So perhaps it's just a convenient way of making a neat and more consistent pattern of buttons?

i have a 39 key Lachenal if that might be of interest. Bone buttons rosewood ends. https://theboxplace.co.uk/product/maccann-duet-39-key/ Let me know of you are interested and I can provide more details.

Better to just enjoy playing it as it is. As Dave says glues are water based and printed patterns are not waterproof. Brush off any loose dust with a soft dry brush and leave it at that.

I plead Occam's Razor

Agreed, but I see the most practical advice as given above and based on experience, is to patch the corners, or do a full rebind. Both are tried and tested, not difficult and if done with decent materials will be long lasting.

Not a good sign of sound advice. 😉

Or possibly tuned an octave below the standard notes?

I'd hazard a guess that the ebay description was written by the AI that ebay now offers sellers. I tried their AI recently when I sold something there and it produced a load of plausible sounding waffle. I think this description is in the same category. Try asking the seller if all the notes play and when it was last tuned. The answers will be revealing. For this price you would expect a German concertina to have been serviced and tuned in preparation for the sale.

The simple answer is no. It’s not just the reeds that are in different positions. The physical buttons are in a different pattern too. So you would have to make a new mechanism and new woodwork to mount the buttons on. It would be almost as easy to make a new concertina!