As far as Diego Costa on a national team goes, I can't believe that Scolari is being criticized for using Fred in the Confed Cup. Fred was the best CF in that tournament, which Brazil convincingly won. Fred was the second best player in the entire tournament. That might have been a one-tournament performance by him, but he was fantastic.
Both Scolari and Del Bosque must have qualms about adding D. Costa, regardless of how well the brute has been playing this year. On the field, Costa is an outrage. Not just occasionally. I can't tell if he acts like that because he can't help it, or because it gets the opponents off their game, or if he just needs to feel all that anger against him to be able to play his best. Adding Costa could damage team morale.
I think Spain can win the WC with Negredo playing in that role. For Spain, the key won't be who they pick, it will be leaving coach-killer Torres off the squad.