Why wouldn't Kirk get back his seniority based on his prior years as a Captain? I could see it if he was demoted to Commnader and then promoted back to captain.
Well, if you watched the earlier films, I think this was mentioned in TMP, that McCoy told Kirk Captain was more his rank then Admiral. Don't you remember him desperately trying to get the Enterprise back from Decker? I think he never went back to Admiral because Captain suited him more. This was also confirmed in Generations when he told Picard to never let them promote him or remove him from that chair because that's where he can make a difference.