A Gunslinger is up against an Imperial commander and his lackey. The Gunslinger shoots the commander, and the commander uses Imperial Valor to hide behind his lackey, who takes the hit.

Next round, the Gunslinger shoots at the lackey and wants to use Spitfire to have the commander take the second hit. Does the commander throw the lackey in front of the second hit as well?

I would say yes.

As GM, In the case of using Imperial Valour against the Spitfire talent as you described i would probably flip a Destiny Point. Its an expensive talent and i prefer not to make my players think I'm a Ass, and the DP is essentially free GM fiat.

I agree with 2P51. Yes it is an expensive talent, but Imperial Valor is meant to make encounters meaningful.

After a while, many player talents end up costing the same as Spitfire. The talent is good (great) enough, don't make it super.

BTW, I would let Imperial Valor work in the OP's post. Just not with the original targeted lackey being the one to take the Spitfire hit. (Target Minion 1 with Spitfire to hit Big Bad. Big bad uses IV to have hit go to Minion 2, not 1 again.)