All right, give him the extra WBL. As long as you buy all the stuff it tells you to, that's fine.
Dragons pretty clearly demonstrate that feat selection doesn't change CR.As for unique Pit Fiends, sure, they exist. You can build any you like - with Leadership, specialized treasure, specialized tactics, Orcus on speed dial, whatever - but the moment you do that they CR goes above 20 and they start fighting some Epic PCs that aren't the focus of this exercise.
If your argument is that a Fighter can't beat a Pit Fiend if it gets to use its abilities or pick its feats, it seems pretty clear you're the one looking for special treatment.