Time wise, It won't make a difference if you do Manoa vs. Diamond Head. Manoa is a very wet, slippery trail often covered in mud and it can take you a while to go a short distance. And imo, they're both crappy, crowded, over-rated trails and you won't find too many locals there anymore. There are much better hiking experiences to be had on Oahu.
if you're leaving at 7:30am on a Thursday, it will probably take you 70-90 minutes just to get to town. If you want to be at Pearl Harbor at noon you'd need to leave Diamond Head or Manoa by 11:30 at the very latest. Not much time to do anything at all. If you don't mind arriving at Pearl Harbor closer to 1:30 you'll have time for a hike.
I'd suggest the following:
North Shore first thing in the morning. You will hit just a short section of traffic heading in that direction and should arrive by 8:15. You could try the Ehukai hike. We've done it with kids and dogs and managed to complete the hike in less than two hours. The view from the top is gorgeous and there are bunkers, just like you'd see at Diamond Head.
Pearl Harbor
Depending on what time you leave Pearl Harbor and when you need to return the car, you could take a scenic drive down Farmington Highway to Yokohama Bay. It will probably be prime time traffic, but these are some of the most gorgeous beaches we have in Hawaii. Or, drive up to Makakilo and stop on Makakilo Drive near the water tank. It's a panoramic view (albeit blocked by a chainlink fence) of the entire south side of the island.