It worked!!!
The board finally arrived yesterday, so I tried it as soon as I could.



After reassembling the unit I got it in the car and it *almost* worked right away. Touchscreen wasn't working at first, so I couldn't insert the new code (language was german) - had to open the case once more, just the top panel, and reattached all of the the ribbon strips that connects the display, and... success! It's now working MFD, wheel buttons, everything as VCDS was already coded for the previous one.

The new board brought me sw3970 pre-installed (already installed 3810 and back to 3970, to make sure the HDD got formatted the right way). VIM is active, as is the "secret" menu.
My previous was 1T0035680C H41, this shows up as 1T0035680D H61. Don't know for sure, but I think H61 is type H (?). Anyway, it was kind of worth it, as I got me an upgrade as well - the navigation seems smoother, but maybe that's just me.

Also, that confirms that the previous mainboard had some kind of flaw, unfortunately. Could it be repaired somehow? It's a shame to throw that away, I might just try selling it on ebay, as a damaged item.

Thanks for all your help.