I have some possible answers for you:

Regarding there being paper residue on the back 
of the cel, yet no damage to the sketch: It's 
possible the cel was stuck to something else. 
Either an unrelated sketch, a genga, or a sketch 
(now discarded) from another layer in the scene? 
(I've never seen it, so I don't know if there 
might be another layer to the cel setup.)

Regarding the key cel indication: There are key 
cels in the middle of sequences too, not just the 
first and ending cels. There isn't anything 
usually special about them (at least not that 
I've ever been able to see), but some people do 
claim that the more difficult positions or points 
of view are sometimes assigned to key animators. 
(I can't draw people well so they all look tough 
to draw to me! ^_^)

About the paper being thinner: The sketches I 
have are on all different weighted paper. Usually 
the paper weight seems to remain the same within 
a given series, but of my two Sailor Moon cels, 
the weight of the paper of the sketches is much 
different. My cel that is more of an action shot 
has a thinner paper for it's sketch than the 
other shot with less motion. This looks like an 
action shot - perhaps they needed to be able to 
see through more layers of the sketches as they 
were trying to animate it? (Mind you, there are 
other differences in my Sailor Moon cels, one 
from the TV, one from a movie, so I might be off 
here.)

Others can attack the other issues. ^_^;; Good 
luck in finding an answer!

Many Sharp Smiles,
--Drac