A few thoughts:
At this point, it won't matter who screwed it up, it's done. You should have 6 lugs per wheel on a QX4.
If the lug stud and nut is stripped, they can be replaced, and it's not too hard. However, based on what you've said, I'd say take it to a local tire/wheel shop.
Basically, they'll pull the wheel, pull the brake drum, and press out the bad stud. The new one will be pulled through from the backside until the plines seat it against the hub, and the brake drum and wheel go back on.
Here's how it works:
https://www.youtube.com/watch?v=A3dM_zg-fYk