I can tell you that I think seperating the cel 
layers is a decent idea.  You WILL risk damage to 
the cel, but if it's done slowly and with lots of 
care you can get minimal damage, and if you bother 
you can always retouch up the cel.  This will 
devalue the cel, and if you ever do resell it, 
you'd HAVE to state it'd be altered and fixed, but 
overall, if you're completely disappointed in the 
cel the way it is, and you want to fix it for 
your own collection, well it is your cel, have at 
it.

Here is an example of a cel my friend bought, 
knowing it was damaged (well layers incorrectly 
stuck together) and then the cel after he 
seperated the layers, cleaned up the two cels, 
applied some repaint and rescanned them.  The 
second layer is actually a correction layer to 
fix his incorrectly painted hair, so it adds 
an even more interesting wrong effect when 
stuck together incorrectly.


 

Uncorrected layer.




Corrected layer.


And I hope these links work. X_x;