Then Seamus was shown as wanting to reconcile later, but Harry only accepted it when he showed up as a member of the DA, not as a friend, but an army member.

Harry was definitely overly-touchy to Seamus in OotP (I personally observed that there were many times Seamus would have tried to reconcile with him throughout the book, but Harry never gave him the chance -- not meeting his eyes when he handed back the Transfig. assignment, rushing past him in the common room when "Seamus looked as though he might have spoken" [don't remember the exact page this quote is on]). However, I did get the feeling he was more happy that Seamus had finally made it up with him than he was when Seamus joined the Army (though that probably made him happy too). It was described as one of the brighter spots of his day when Seamus finally talked to him, but we didn't get anything on how Harry felt about Seamus joining the Army (he was just kind of there, and it was mentioned that it was his first meeting).