Not a snowball's chance in hell we'd give up Nieto alone for Callahan.
Personally, I have absolutely no interest in Callahan, but he seems like a DW player. However, the best you can hope for is basically what you gave up for Clowe last season: two 2nd rounders (and probably a cap dump because we can't afford Callahan as is). He would be a pure rental for us since there's no chance at all that he'd re-sign with us (nor would we give him the outlandish contract he's supposedly seeking).
Let me go find a barf-bag.

Don't even mention Hertl. Jeez.