Personally, I would say not to waste your time worrying about it. If anything, keep your slang down to a minimum because that's where the majority of the differences are, and you Brits have a lot of slang. The problem is, for Americans as well as Brits, is that we don't realize how much slang we actually use in daily life. Even in your examples, no should have a problem understanding any of those except for maybe "skip". Then again, how often does anyone use "dumpster" in daily life. Are you expecting a run-in with a skip while you're here?
Anyway, the fun part about going to another country is the language difference, isn't it? By the way, Dr. Who and Downton Abbey are terribly popular here, and who doesn't like Monty Python or AbFab. So if you run into a snit, pretend you're in the Dept of Insults and channel Eric Idle (then run like mad). And Russell Brand and Ricky Gervais are on TV (that's a telly) quite frequently, so you see, we're not totally unfamiliar with your accent. If worst comes to worst, try pretending you're a BBC newsperson (they're always fairly understandable to us - well, to me at least) or Julie Andrews.
As with anywhere in the world, if you stay away from the dodgy parts, you'll usually not run into any problems.
Safest/dangerous states? Stay away from the West coast states (volcano, earthquake and tsunami zones, not to mention the killer whales). Stay away from the NorthEast coast (hurricanes, New Yorkers, and the Mafia). Stay away from the Gulf coast (again hurricanes and Latino drug gangs (haven't you seen "Scarface"? and the BP oilspill). Stay away from Southern and Mid-West states (tornadoes, country music, Honey Boo Boo). You're probably safer visiting Canada.
Photography law? No taking or distributing pictures of naked children, otherwise knock yourself out - it's a free country.
Dress code? If you've kept up with the latest hip-hop videos, you'd know it's bootie shorts and string bikinis for the ladies, and whatever for the guys.
Traffic Safety: Don't jaywalk and no crossing against a red-light (red man)
Gun laws: I'm not sure but I think they make you carry a gun in Texas and Arizona
Otherwise have fun!