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What kind of plastic? If PLA 3D Gloop PLA works amazing. Otherwise hotglue, or my suggestion is don’t do single key pcbs / handwiring (ad) I make flexible pcbs for curved keywells (specifically used them on my dactyl keyboards I’ve sold, hundreds in the wild without any issues! www.skree.us or etsy.com/shop/thebigskree if you’re interested in that. (I ship internationally but it can sometimes take a while to arrive).
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If the micro controller works with the software (ZMK / QMK / KMK etc) then yes, it works. Both of those would work.
Not that I’m aware of but the DRC systems should be able to make it pretty clear if there’s track issues.
There are circuit / electrical simulators but that’s a very complicated solution for keyboard designs that tend to be simplistic.
Matrix circuit designs are not much to be worried about. If you had a design in question I’m sure someone including myself would be willing to look at it.