I believe the "eye catch" is the sequence where 
the commercial break goes. It's not an actual 
part of the episode, usually; rather, it's a 
stand-alone little bit of animation. Like in 
Trigun, it's Vash in reds and then blues; in HYD, 
it's Tskushi in the bathtub; in YYH, some of the 
eyecatches are the group as chibis, and they pop 
the words as they fall, and so on. Sometimes the 
eyecatches change over the course of the season. 

I *think* ---and I may be wrong here--- that key 
cels are any cels drawn by the key animators, not 
just the A-1 and end cels. So, for instance, if I 
draw A-1, A-5, A-10, and A-16END, those four cels 
would be key cels. My inbetweeners would take 
care of A-2, A-3, A-4, A-6 and so on. I *think* 
that A-1 and end cels are the easiest to identify 
as Key Cels, since, to the best of my knowledge, 
those are always drawn by Key Animators... but I 
believe that those aren't the only key cels. If 
I'm wrong, please correct me. :o)

Eyecatch cels can have multiple layers, just 
lilke any other cel can have, as you said-- A1 
end B1 end C1 end, but by the same token, they 
can also run something like A5end B7end C2end. 
Sometimes they're animated; other times, it's 
just a static image. 

-Cres