That photo of killington is very disheartening!!
Killington is a huge place, I can fully understand the management not wanting folks skiing there while it's closed. In my personal experience, cell phone service can be spotty on parts of the mountain. If someone gets hurt, it might be difficult to reach help.
Personally, I wouldn't want to ski there completely alone, without ski patrol. If you get injured, you also risk infecting first responders.