Thanks a lot for the reply. I've been going through each of your suggestions.
Firstly, I've tested using a few different runners, including caffe-9.7, ge-proton10-15, wine-ge-proton8-26, kron4ek-wine-10.14-amd64 and a couple of others. In every case, it fails on the original bottle but works on the new. I've got what I think are the latest versions of dxvk (2.7.1) and dxvk-nvapi (0.9.0); but not the latest vkd3d (there seems to be a problem with the download of this in bottles at the moment. I have 2.10.).
By default, the game uses dx12. But if I launch it with -dx11 then it uses 11 instead. So I tried disabling vkd3d and launching with that command option as you suggested; again, it fails on the original bottle but works on the new one. When using dx12, there are heaps of messages in the terminal (mostly identical, but with that failure near the end only for the broken version). Whereas with dx11 there are very few messages in the terminal. The broken version gives me 00b4:err:hid:udev_bus_init UDEV monitor creation failed. (every runner gives me 002c:err:wineboot:process_run_key Error running cmd L"C:\\windows\\system32\\winemenubuilder.exe -r" (126). - but I seem to get that all the time on every app even when it works fine. So I assume that is irrelevant.)
I'm not sure what you mean about 'can't download the newest Mono'; because I'm not sure when / what might be trying to download Mono. Do you mean bottles itself, or the game, or something else - I'm not really sure how this stuff works exactly. In this case, I installed mono within the bottle manually by downloading and msi and running it in the bottle. I wasn't able to find it in the dependencies list inside Bottles, so that's why I did it manually. But I got the link to the msi download from the bottle dependency list on github. In both of the bottles, the windows uninstaller list tells me that I have 10.1.0 of the Wine Mono runtime.
All the mono stuff looks right to me, but given that I don't know what I'm doing, I reckon its quite likely that the root of the problem is related to that stuff. Maybe I didn't install it correctly or something.
I tried deleting the cache, and that didn't work. I also tried copying the cache from the working version to the broken version; that didn't work either.
Anyone, thanks again for your thoughts and info on this. I feel like the underlying message in what your saying is that I should just accept that it works in the new one and not the old one - and take that to be the standard practice for how I should do it anyway! Which isn't exactly what I want, but perhaps it's the best answer I'm going to get.
I also have dual-boot with Mint, because I expected to be using Windows for gaming. It turns out I have never needed to. Every game I've wanted to play has worked on Linux; and so my Windows partition has just sat idle.