The short answer is: she's crazy, she's playing pretend and it's a plot twist.

Why all of a sudden? Maybe for exactly the reason she says: Komaru forced a final confrontation and Monaca can't be bothered going through the motions. There's no longer any reason to keep up the pretence.

As far as her participation through Gekkogahara goes, it actually explains why Monaca does nothing to incite despair in the group (short of what she called "a harmless prank like in the old days"). If anything she's downright encouraging towards Naegi and the rest.