We have a park pass/military pass, if we park in the lot and go hike the trails, where can we park in case we miss the 6pm deadline so our car is not locked in?

Unfortunately there is no place to park that doesn't have a gate on it, at least near the visitor center. The Hornbek house doesn't have a gate on it and it has a wildlife trail that eventually connects to all the trails, but it is a 1.6 mile hike back to your car.

I never noticed any gates there so idk but the staff is so nice stop in and ask they will help

The quick answer is, yes.

If you look at a map of the Park Hill see some normal County Roads that border the park and you could park on one of those and walk in or hiking