I think there are 2 alternatives that are at the top of the most likely scenarios for the change in Carlos:
1. Carlos has questioned his initial reactions to Battle Ground as well as his loyalty to the White Council. You can imagine Carlos re-thinking things in light of (1) the Merlin declining to immediately address/avenge the fates of Yoshimo, Meyers, and Chandler, (2) the Knights of the Cross (active and retired), Father Forthill, etc. still standing with Harry, and (3) how Harry pushed the Accorded Nations to help normal people affected by the events of Battle Ground (which surely must have become known to the White Council). So Carlos approaches Harry with an open mind, doesn't see any sign of black magic, gets treated firmly but fairly by Harry, and finds Harry still on board with dealing with Drakul. So Carlos is willing to give Harry the benefit of the doubt, to extend 'trust to a degree but verify,' and to work with Harry where it doesn't cross his obligations and remaining loyalty to the White Council.
2. Carlos is working Harry, at the behest of the Merlin. The Merlin has no eyes on Harry, has to be concerned about Harry's increasing power and prestige, positions, upcoming marriage into the White Court, etc. and has decided to keep his perhaps enemy closer. So he's sent the person that Harry has a long history with, that Harry has considered a friend, and that is low enough in the White Council hierarchy/structure to be less suspicious than, say, Luccio. The 'Merlin won't help with ' card adds credibility to Carlos, giving him a sympathetic motivation for approaching Harry that Harry will be pre-disposed to believe. Given how Carlos views Harry's lack of loyalty/friendship to him, and how Carlos knows he was lied to, misdirected, and backstabbed by Harry (in short, intentionally played for a fool by Harry), I can see Carlos being undisturbed by returning the treatment. Me, I think this is the best explanation for the sharp turn by Carlos after the end of Battle Ground, especially after accounting for the enjoyment Butcher gets in torturing Harry.