I've tried the freezer method as well with a 
terribly stuck cel. Some little corners did 
managed to come off. But on the whole, it's still 
stubbornly stuck.

I've also left the cel in overnight. Nothing, 
terrible happen.

When freezing it, the cel curled and when thawed, 
it straightens itself flat again. Would like to 
ask, if I hold the sketch while freezing it and 
the cel starts to curl, will the paint come right 
off from the plastic sheet?

I've tried to peel the cel from the sketch and 
resulting some paint loss, it hurts. I didn't 
dare try that again, unless I'm pretty sure it 
wasn't that badly stuck.