I don't think you are asking the right questions.
There never was a single doubt about who own the 
copyright to the picture represented on a cel : 
It's obviously the copyright holder of the anime 
and not the owner of the cel.

But that doesn't mean that the owner of the cel 
doesn't have some right himself. That only mean 
that he can't use the cel in a way that would 
break the copyright of production company.

You asked in what way a fan cel could be illegal. 
That's also obvious : As soon as you make a 
picture (whatever the media. Even a CG) that use 
copyrighted characters and sell/trade/exchange 
that picture (ie: it isn't for personal use), you 
are breaking the copyrights.

By the way, if you look at the credits of an 
anime, you will notice that it isn't copyrighted 
to a single company but several : The producing 
studio, the "sponsors", perhaps the animation 
director...
To make a produce based on the anime, you need the 
agreement of all copyright holders, not just one 
of them.

As I said many time, there is really two things :
what is legal and what should be done. (or if you 
prefer, what is ethical).

Concerning legality :
It is illegal. In the best case, that mean that 
the copyright holders may tolerate it as long as 
it doesn't become a problem to them. In the worse 
case, it mean that the copyright holders didn't 
came around to take legal action yet.
Even if the copyright holders don't do anything 
and let it happen because it doesn't disturb them, 
it doesn't give you any right. They may at any 
time change their opinion and decide to take some 
actions without any notice. (Ie: they are not 
granting you a right to make a fanart. They only 
let you do it as long as it isn't a problem).

Concerning what should be done:
Well, I guess I explained myself lengthly in one 
of my other message : If you really are a fan and 
you do fancels for fun and not to make profite or 
cheat other people, then you could use pictures 
that are not in someone gallery.
When you are free to pick any pictures, why choose
one of the few that you know will annoy people and 
cause problems ?
And if you absolutly have to make a copy of 
someone's cels, why not ask the owner if he mind 
it or not ?
Also, why not make your fancel in a way that it 
can't ever be sold as a real production cel ?

So why copy cels that you know will cause problems 
when you can as easly copy other ones ?

When a fan artist copy an expensive, popular cel 
to sell it and do it in a way that may lead people 
to confuse it for the original, that cast serious 
doubt about the intent of the artist.
And when the fan arist feel the urge to write "A4" 
in the top-right corner of the cel, there isn't a 
single doubt left about his intent.

Which lead back to the legality question : If 
someone buy a fan cel, thinking it's a real cel, 
he could lay a claim against the seller and the 
person that made the cel. That has nothing to do 
with copyright.
In fact, if someone see the cel, think that it may 
be misleading, then it could denounce the seller/
artist even if he doesn't buy the cel. And it's 
not because nobody did it that nobody will do it 
or that it make it "ok".

Cheers,
Yann Stettler