I don't doubt that you can put a 12mm lug bolt through the hole made for a 14mm lug bolt.
However, the mating surface of the lug ball on the wheel will be the wrong radius. A 12mm ball won't surface properly on a 14mm socket surface, so the bearing loads will be all wrong.
As an engineer, I wouldn't bet my life that it will all stay together.

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KenP

1996 S500 W140 Coupe