Sure, direct sunlight will effect fading but 
humidity is the big problem after that. Its the 
same reason museums will use climate control for 
some of their collections. The lines are powdered 
ink photocopied on acetate, change in water 
molecules in the atmosphere will dissolve (over 
time) loosened particles (lines) on the cels. 
Toei must of had a really cheap machine put the 
lines on their cels to have such major fading. 
Cels in frames have a better chance of fading 
than those in books but they will still fade in 
the books. I say go ahead and frame the ones you 
want and don't worry about fading. If it happens, 
it happens! At least you'll be able to enjoy the 
cels everyday without digging through the books.