I was only there once or twice in off hours. I think I was there once on a Sunday (normally closed all day so only open to after hours members) and once in the evening. It was quiet as I recall but I guess I’ve not made use of it enough to have an idea. It’s not overly busy in the after hours.
W.r.t. alcohol, the rules forbid eating and drinking in the library, but water is exceptionally allowed. I don’t know if they review the video without cause, but if someone breaks the rules, their after-hours access is terminated.