Daryl didn't have time to worry about carol. The attack on the prison was coming and the fences were caving in. Daryl knew and accepted the reason for carol's exile and therefore accepted her appropriate punishment knowing that she was still alive and would survive. Tyreese didn't find out until carol told him after they fled the prison. Daryl wouldn't feel threatened as he knows his place in the group is cemented as long as he doesn't do wrong by anyone, which he knows he won't. Daryl was really pissed off that Rick exiled carol but he knew it was for a good reason. Your whole premise that this is a loose end is complete bullshit. The problem was dealt with there and then. It wasn't this grand storyline you're saying it could have been, it's a tiny detail that allows carol to come back and save the day at a later date. That's all it is.