by RHPDaddy
I didn't see any where in the instructions that you need to discard them. I believe they remain in your hand unless otherwise stated.
Please let me know if this is not the case.
Defending against an instant event enemy is carried out just like attacking an event enemy. You actually play cards out of your hand onto the table (and can use their abilities) and subtract their strength from the strength of the attacking enemy to determine the damage applied to your Home Realm. So after the resolution phase, when you play the "end of combat" phase, you discard all unwounded unit cards to the discard pile. Just read page 10 of the core rule book carefully.