HHN does not affect IoA. In recent years the event has been exclusively at the Studios...not IoA so you can enjoy the park until 7pm on those nights when HHN is staged. The Studios closes at 5pm on those nights for set up so you would have to leave at 5pm...unless you're staying for HHN of course.
Now...this is just a rumor but...I've heard that HHN will extend to both parks again this year (first time since 2005) but it's way to early to tell if this is true. I don't really believe it but stranger things have happened.