It's true that the trophy for Easy completion should have unlocked already. Was this your first game through? If not, perhaps you unlocked it already.
Either way, don't bother doing an Easy run just for those trophies. Survivor is the hardest difficulty. There are trophies for completing it once, and then again on New Game +. The minimum number of games required to collect all the trophies is 3 (so long as you don't unpatch and cheat). It won't feel like that many when you are done, because the game is so great.
Yes this is my first time through. I finished it on hard because I thought it would be similar to uncharted to where you have to complete hard mode to unlock the most difficult mode. So if I complete the game on the new game plus mode I should unlock the trophy for the easy play though?