Knowing that the ball was gold gives you Bayesian knowledge about the boxes behind the door, since the prior probability of the host pulling a gold ball from a 6-gold door is different than from the 3/3 door. So you have to multiply Monty Hall probabilities and Bayesian probabilities together.
That assumes the host pulled a ball at random, of course, and not a deliberately gold ball.
I agree that OP is in the best position to report the crime to the police - they are closest to the police station, they have video evidence, they literally know who the thief is - but it should not be their responsibility! OP has done nothing wrong and there are no measures they could have taken to prevent this crime (other than not shopping online at all). If OP gets a police report, OP is taking up the task of being the victim, and then BestBuy has no legal obligation to refund them at all, other than out of the kindness of their heart. Rather, BestBuy is the victim in this crime, same as if the item was stolen off the shelf at their warehouse and scanner records forged. It is their responsibility to file a police report, if they want the numbers in their system to add up. Only then could they ask OP to kindly provide the video evidence to help them out, and they'd be lucky if OP would give it to them, having no obligation to do so.