GM hacked?

Checkmate.

OK now the real question is: did a GM put his info and password in the marquee (and if so, why would he leave his information there?) or was the account hacked, the marquee changed with the information so people could see it was real, and then the chaos happened?

Before we can answer that, let's ask;

Is there any reason that a GM would put their information up there? Given Nexon's apparent recent stressing of account security, it's safe to say that's a no. Ergo, the account was most likely compromised before the info went up, not the other way around.