So what happens if a pc players uses an event key now?

or are they not usable when imp come out?

Event keys are going off as Forest of Thorns. Every Imp week always gets associated with a kingdom somewhat similar to it even though they have no kingdom. You can tell by the troops that come up to the left before opening an event key as well as the background image of an Imp event week. The background image behind all chests every week is the image of the kingdom for the event.