Hello,
Just use pure (90) alcohol (not wine! :) : this
will remove the ink quite easly.

Take those stuff you use to clean your ears 
(sorry, not the faintest idee how it's called
in english), dip it a little in the alcohol
and then remove the ink that smeared on the cel.

Just be careful not to go over the actual lines
as it would remove them also. If you take your
time, you can get a pretty good result.

Yes, if the problem is between the cel and
the paint, you can't do anything. But it won't
happen : when a cel is hand-inked, it's always
on the top. And if the lines are flashed, they
can't smear like that as it's not a ink that is
used.

Cheers,
Yann Stettler