I'm not sure about all the details, but taking what you said in the OP as true, here are my thoughts:
A cop doesn't need probable cause to call a drug dog to a car. All he needs is to have the dog there by the time he finishes up his "investigation" of the vehicle (i.e. talking to the passengers, running a driver's license check, etc.). On the other hand, he can't unreasonably delay his investigation in order to give the drug dog time to get there.
Refusing to consent to a search does not give a cop probable cause to search your car. If a cop used that as an excuse, anything he found would be inadmissible.
Once the arrests are made, the cops are free to tow your car to the police impound and search it, regardless of what the arrests are for. But if the cop was unreasonable in his investigation, either by wrongly claiming probable cause or by unnecessarily delaying in order to give the dog more time to get there, then the guns would probably be inadmissible as fruit of the poisonous tree.
If the weed was in BW's backpack, it's conceivable that he could take all the blame for it. And if it was stashed somewhere in the car (glove compartment, trunk, etc.) it's conceivable that Tatum (the driver) could take all the blame for it.
Having an open container in a car is not illegal in Tennessee. Unless the laws of Knox County have changed since I left, the only reason this would be an issue is if the cop thought Tatum was the one drinking out of it.
Of course, Bruce knows there were two guns in the car, so a bunch of legal technicalities might not keep these guys on the team.
Anyway, I think that stuff is correct, but once you pass the bar exam most of the information goes straight out of your brain.