It is a moving window and is configurable by number of days 0-7. Events older than the configured time are removed. If ASUI crashes or is restarted it will lose all event data since it isn't saved.
There is only a signal that is sent whenever the power button is touched, short or long. MCE calls a systemui powerkyemenu_open method when the button is short pressed and the TODO page documents the problems I had when trying to convert from the signal to this method call. As far as I know there is no signal or method call when the button is long pressed, MCE handles the shutdown internally. But if you know of a way to read the raw power button events...
BTW, why do you want ASUI to handle shutdown? ASUI does listen for the shutdown signal and switches to its charging UI.