Celeste's vampire motives was just a cover for her real motive, like she said, she wanted out more than anyone in the cast, she just finally got broken down to the point where she acted on it. Everyone else saw hope at the discovery of Alter Ego that they could get out, but Celeste didn't get that feeling at all.
Also, Gundam pretty much explains it, he set this all up so that everyone had a chance to get out, but his personal code prevents him from killing himself outright (which is why he challenged Nekomaru), giving himself up in a trial wouldn't be any different from that.
I kinda doubt it, outside of actual brainwashing (which Junko doesn't start until she has the First Killing Game to work as a brainwashing program), Chiaki doesn't seem to have much in terms despair going on. She says that having a talent is a limiter, but she doesn't seem to mind it all that much.
Meeting someone like Komaeda though, someone who regards normal people as on his level (on the level of trash), would be a really bad person for Hajime to meet in his current emotional state. I could definitely see that if he and Hajime bumped into each other that the resulting meeting could push him into deciding to go in the IKP.