Maybe the regulator you got from BMW is faulty??? An alternator do not just fail like that. I was lucky. My 325i has the Bosch alternator. And even though I have the Bosch, I still had to get the regulator from the dealers as Bosch SA did not even have stock, BMW did.
It's a pity that you already bought the regulator. Have they tested the new regulator separately for you?
Is the air duct the same for the Valeo and the Bosch? Just thinking...