The freezer would actually be a good place to 
cool down cels.  Colder air is drier than warmer 
air (good ol' physics), so moisture is not a 
problem.  (hence freezer burn, why people get 
sore throats more in the winter than summer, 
etc. . .)

Since it is winter (in this part of the world at 
least), the garage would be an excellent 
alternative to the freezer if there's no room in 
the freezer.  (Obviously, the cel would not go in 
the garage/freezer bare; in cel bag.)  Whenever I 
cool a cel, I try to remove as much of the sketch 
as I can.  Then I would repeat the cooling and 
peeling process until the sketch comes undone.  I 
know someone who did this for several weeks until 
a sketch became unstuck.  I haven't had to do it 
for that long, but don't expect to get all the 
sketch off in one try.

(Yes, cold can warp a cel but the cel will unwarp 
itself after warming back up.  If one is very 
concerned about warping, then simply place the 
cel [in its cel bag] between two pieces of 
cardboard and place a paperweight on top [in a 
dark place of course].)

If one doesn't care about the sketch, water will 
separate paper as well, but dunking it in water 
(as was suggested to you) would be overkill.  
Just grab a paper towel, soak up some water, and 
just dab the stuck areas with water until the 
sketch is wet throughout its thickness.  The 
sketch should just peel right off then.  You 
don't need to wait a ridiculous amount of time 
either, maybe a few seconds at max.  If part of 
the sketch still won't peel off, just dab on a 
little more water in the offending area.  Yeah, 
water can warp a cel but only if you leave it on 
there for a **long** time.  After peeling off the 
sketch, just simply wipe off any remaining water 
with a paper towel or cloth.

(Yes, water should not remove paint on cels, but 
you may see some paint residue on the paper towel 
after removing any excess water that may have 
gotten onto a cel.  It really is no more than 
what would appear on your [photo] gloves.  Paint 
on backgrounds is another matter entirely and 
water shouldn't be used to remove stuck, non-
matching backgrounds since the bg paint can bleed 
onto the cel.)

I've separated a lot of sketches using the above 
methods (mainly water since AIC has an agenda 
against providing matching sketches, grr), and my 
cels aren't warped because of them.

Obviously, if you don't feel comfortable doing 
any of this, then don't do it.  If necessary, see 
if you can try it on a cheap cel first, or you 
could try sticking together some paint and paper 
first and try the various methods on that.