What I've heard from cosplayers is:
- absolutely take a bag, but something that'll keep artwork from being bent or warped if you end up buying any.
- check the cons rules on if brining in food and drinks are allowed; if they are, then it's a good idea to bring snacks that are easy to transport
- being a normal (polite) person is definitely okay! If people are in cosplay, though, and you wanna get pics, then talk to them first and be understanding if they're not feeling up to a pic. Costumes can get heavy and uncomfortable after a point and they might be tired. So basically just be nice and don't be rude.
- I'm not sure about the last two. You might want to ask the people who run the con if it's okay to leave early or not (I'd assume you can, but getting back in will be next to impossible if the con's really full of people and the event's over-full).