Hey JP!

I'm sure someone with a more extensive knowlege 
will come along and answer this better, but I'll 
give it a shot.

1) I do notice a lot of my Tenchi cels have the 
remnent of paper stuck to the back, but the 
official reason is still that the sketches were 
removes so that fake cels couldn't be created 
from the original sketch.

2) The black paint on the back of a cel is for an 
effect called backlighting....like Tenchi's sword 
of light when it glows that is light being shone 
from behind the cel and so the cel is painted 
black on the back so that the paint doesn't look 
transparent.  I know this has been explained in 
the archives, ^_^; I'm just terrible at searching 
them, but you might want to check there.

-Emika >^.^<