Oh, what a disappointment for you. So sorry you are going through this. Good for you for posting to ask the hard questions, though - you'll find what's right for you in the collection of opinions here... And for MY two cents:
I believe the matter of ethics the first responsibility of the breeder to do what's right for the breed with the highest minded intentions possible... If you're going to take on the perpetuation of a breed, then you MUST do it right to honor the breed, and the collective effort of all who strive for a pure line. If you want to keep these birds as pets, that's completely wonderful, but if you intend to sell hatching eggs, then it is your responsibility to start over with birds from a reputable source - again, through Maran's society recommendations, or recommendations from somebody well established in the breed, like the moderator here Wynette...
The presence of another strain should be a very very deliberate thing - not the accidental discovery on the other end of the project... and if it happens it should never be sold as or claimed as BCM... because it isn't... to perpetuate that is unethical. The truth is that with the presence of another strain in your chicks, anything you do to correct it will be complete chance, as will the resulting offspring. Lucky and ethical are two different things.
I'm glad to know there are reports of Wheaton lines showing up in a particular breeder's stock - makes good sense to avoid that stock if it isn't true.
Okay, off the soapbox... air is thin up there