I would stick to a round hole personally, even if it has to be bigger. Corners make stress risers and can allow cracks to start easier than circles.
Mine have been enlarged for a few years now and haven’t needed any strengthening.
I’d possibly look at the stalk for that, sounds like a wire is contacting only when the rear wiper is switched on. Was there not some guide about stripping them down somewhere? I know there’s one for the lights stalk. Would cost nothing to open it up and see.
Welcome along, it’s great to see overseas members. I’ve heard a bit about TUV, is it the case that it’s oem parts or only mods which have a TUV approval that are allowed. No way of getting unapproved things approved?
As above really. ACF-50 and socks will help but if it’s just a track car you should be fairly ok as I’m guessing there’ll not be many, if any, road miles through winter. My old ones corroded quite badly even when coated in ACF-50 but they never seized.