You could take it to BMW - but I wouldn't ask them to see "IF" there are cracks. Id tell them you have been told there are cracks, and you would like a second opinion and MAKE SURE YOU GET THEM TO TAKE PHOTOS!!!. And that you would like to know if they are aware of the subframe issue (id approach them on that angle because you want them on your side). As the BMW dealers may go on the defensive.
Then ask them how familiar they are with the issue and if they are willing to sign a document like ive mentioned above. So you can go back to where you had purchased it.
I personally would go to bmw, get pics and a statement from BMW & your mechanic, then seek some legal advice. If you like i can help you with the legal firm, I used the same solicitors that CortinaD? did, and they are very professional and they have dealt and are very aware of this problem. Id ask what your chances are of returning the car / have the dealer fix it or have BMW fix it.
TBH you need to be smart about this. Because your between a rock and a tight place. Theoretically your not legally allowed to "knowingly" sell a non-roadworthy car, yes there is not much chance the buyer would find out. But if they did ive been told the penalty is quite hefty.
Your only other way out is to trade the car in for something else, which of course will leave you well out of pocket. :/