"Does Denver get creepy or weird or crowded or absent after 4pm or anything?"
Nah. I work down here, and it's generally alright in the downtown area we'll be in. Being a Saturday, you'll actually start to see traffic increase as evening comes along, if anything.
(And yes, Across the Universe is awesome.)
Also, if someone's having trouble with transportation to Denver (you had mentioned a couple), I think we ought to prioritize getting them a ride over those who have nearby bus service, etc. Anyone disagree?