Yes, he made peace with his brother, but then he had to get out and do something. Even though it was only taking a walk, it wasn’t sitting at Louie’s bedside and being comforting. I think that’s why not having a dramatic reaction to Stella and Flack’s situations was in character. Like I said, if TPTB had shown any of Stella or Flack’s recovery, Danny would have been able to do something. He’d help out his co-workers/friends.
But Danny does have a dramatic reaction. He falls crying into Macs arms, he crys at Louies bedside. I think the big difference with RS,RD is that he feels in someway to blame for his brothers situation. There is a lot of things that have been left unsaid between them and Danny is concerned that he'll never get to say them.
I'm not saying that he had to lay weeping at Flack's bedside, i don't think their relationship is like that.
In heroes Danny isn't working Aidens case. He may trust Mac but his friend has been murdered (most likely raped) but what could he possibly do? That wasn't his case, he was working on the murdered marine, yet we still see hot headed Danny, Mac telling him they are doing all they can. Flack steering him away from the topic and back to the case they are working on.
In CoTP he is working the case. He can do something to help his friend. I think you are right about TPTB, they went the wrong way about things to be honest i can't even remember Danny asking about Flack in CoTP, Stella does, Lindsey does, even Jane Parsons knows how he is doing as she gives Mac an update. Hawkes goes to check on his CAT scan (although he is a doc so i guess he was being practical) The only thing i really remember Danny doing is searching for survivors. Danny doesn't find Don, he sees a hand in the rubble. It's the guy that Flack was telling to get out of the building that helps the rescuers get to them because Mac tells him to make some noise and alert the rescue team. I know i'm just splitting hairs and that you can't show everybodies reactions. I just felt that Danny was let down and pushed into the background in an episode where alot of people felt he perhaps should have had more to do and say. We had been led to believe that Flack and Danny were close. I guess some of the audience felt let down by Danny's reaction to it.
Sorry if i have just rambled.