I thankfully have no cels that are stuck to their 
backgrounds (although some apparently used to be 
before they reached my hands). But it sounds like 
you need to get this one unstuck.

If you are going to separate them, I would do so 
by sliding a knife between the cel and background 
and cutting a tiny bit off the surface of the 
background. (However you take them apart, the 
background will be damaged - this seems the way 
least likely to damage the cel.) Once they're 
separate, I would somehow remove the remainder of 
the background from the back of the cel - keep in 
mind that there's still paint on the part that is 
stuck to the cel. You might try a cloth dampened 
in distilled water - but I don't know if this 
might not make the bleeding worse by getting a 
watercolor paint wet again! O_o  Once the 
background paint is out of the equation, the bled 
on parts of the cel shouldn't get worse.

As for what to do about the bleeding itself, I 
don't think there is anything you can do. The 
only conceiveable way to fix it would be to 
remove and replce the paint on the cel. . . And 
needless to say, I DON'T recommend that!! ~_~;;  
But whatever you do, do it slowly and take your 
time!

Can anyone offer better advice than my 
speculation?

Many Sharp Smiles,
--Drac