As far as the sketches go, you can just press them
out, but the cels, that is very hard to say even
because any heat or excessive pressure may damage
the paint. 

I one time had a bent cel and after leaving it in
my cel book for so long, it ended up straightening
out.  
What you could do to at least uncrease some of it,
is to just put an acid free board or paper behind
it and just leave it in the cel book.