Welcome to the forum.
If you're getting muck in past the air filter, there's a good chance it will destroy the motor. If you have any documentation that proves you've had the vehicle back @ Nissan for this, hold onto it, because if your motor goes west, not only is that expensive, but it takes a long time to get it fixed and you don't want to be paying for it.
I'd be onto the service manager about it. And the guy who arranged the sale. Be amicable, because honey does attract more flies than vinegar, but be persistent.