1) First I've heard of this "official" reason, but 
I suppose it makes sense.  ^_^

2) I don't think that a backlighting effect 
applies to the cels I have.  I say this mainly 
because the black paint does NOT go all the way to 
the edge of the image.  There is a border of 
original paint pretty much all the way around.  So 
if it was backlit, the edges of the characters 
would "glow" (unless that is part of the effect!). 
 Instead, it looks like someone slobbered paint on 
there with the intent of covering most of the 
image w/out worrying about being meticulous.

And your mention of the sword effect brings up a 
question I posted a week ago.  I have a cel of 
Tenchi in battle gear with his light sword.  I 
thought the sword would be done with a 
backlighting effect, but on this particular cel, 
the area where the sword blade should be is 
painted black (and Tenchi himself is painted 
normally---no extra black paint!).  Mysteries, 
mysteries.....   ^_^

JP