What time does the park close that night? We've stayed in the parks for 60-90 minutes after the posted closing time and of course by then the area in front of the castle/hub and Main Street are pretty empty. BUT....realize that it's also dark outside and while there is lots of lights on the buildings and such, it takes a very good camera and usually a tripod or something to keep the camera very steady to get a photo that doesn't have the lights all blurry. We've managed to take some great shots with the camera resting on the top of a trash can (thank heavens they're semi-clean, lol). There are Photopass people out and they have tripods, but it's hit and miss whether they know how to shoot at night.
An alternative.....make a MORNING reservation for Crystal Palace for before the park opens (watch that it's not an EMH morning). We do this for our first morning every time.....8:05 is the earliest ADR and the park doesn't open until 9am. Arrive about 7:45-8:00am and you tell the folks at the front that you have an ADR (always have your confirmation number handy....I have been asked for it once). They let you in and the park is bathed in sunlight for great photos, but the only people around are those with ADRs. We've always been able to wait a few minutes and get some time with either no one or only one other family in the photos. And there is something ultimately special about walking down Main Street ALL ALONE. Of course, we never walk, we have to skip and dance like crazy people, but I'm sure others watching understand (I hope). Unfortunately there aren't any Photopass folks out that early, but someone would probably be willing to take your family photo for you, maybe in exchange for you doing theirs, lol.