Sounds like a bunch of guess work and throwing parts at it. I would 1) check the codes and see what's going on. And 2) have someone that knows what they're doing help you.
From the sounds of it, an IAC isn't going to keep it from starting. It will keep it from idling on its' own but usually won't cause a no start. If it won't idle a simple cleaning with some TB cleaner will more than likely fix it. But the starting issue, my guess would point towards the ignition switch down on the steering column. They go bad more than people think. And once you pull your hair out trying to diagnose it, throw parts at it, etc it ends up being the ignition switch. But like I said, you would be much better off having someone that knows what they're doing help you. Even if you paid a shop, in the long run you'll probably save a couple bucks and have a few hairs left on your head lol
