I have a couple cels like that too. It would do 
more harm than good trying to separate the 
layers at this point. I can almost guarantee 
that the paint will come up and thats much worse 
than a little humidity.
   As for long term damage, The cels I have that 
do that are about 6-7 years old and it doesn't 
seem to have affected them at all. I worry more 
about light and line fading...



pale