Check in online then you won't have to worry about the airline giving away your seats if you get to the airport later than expected.
On most mornings, coming from Ko Olina, traffic should be negligible after 8-830am. You might hit some slow down near the H1/H2 merge, but by that time, it should be flowing nicely. The next slow down occurs after the airport at the Middle St merge so you won't hit that at all.
2-2.5 hrs should be more than enough time to drop off your rental car and checked bags.
One thing though, if there's one significant accident anywhere on that stretch from Kapolei to Aiea, good nite! You'll be squeezing cheeks cursing me!