After looking again at the pics provided by the OP, I think he might have to accept the fact and eat the cost of replacement on his own. S&W's offer seems fair, if not generous.
And the reason I say this is the pic of the ammo in the box. Ammo makers, even Federal do NOT mix differently headstamped brass in the same box.
Since there is at least one case with a different headstamp than the rest, it is clear that, at some point, the ammo was "tampered with". Now, it could have been a factory screw up, but I don't think that's likely.
The age of the ammo in this case, works against the OP. Not from a "old ammo is unsafe" point, but from a "we have no idea what was really in that box" point of view.
Clearly all the rounds in that box were not identical. Now we don't know, and can't proove that they were all factory ammo. The OP could, through no fault of his own, be the victim of product tampering.
Lots of ammo is sold in factory boxes that isn't what the factory originally put in that box. The mismatch of case headstamps prooves something happened to that box of ammo after it left the factory. Federal is not responsible for that. Nor should they be.
It's sad, but I don't see the OP having a good case.
All else being equal (and it almost never is) bigger bullets tend to work better.