If it just all the sudden doesn't start up anymore without any sign, then im not so sure its the main relay, normally or what happened to me and anyone else I know that this happened to it generally will start up and then either because of the really hot day (raises resistance of the electrical flow on the wires to the relay) or because they went on say a decent little road trip and the engine just got that hot, then after shutting it off; like to get gas or something. Around 2 mins later when they go to start the car then you can't get it to start. However for it to not start at all and you replaced all those things my only other guess of the top of my head is TIMING. when I accidently set my timing 90 degrees off my motor sat there and cranked and cranked with no problem but it never started or even thought about it. Which is when I started looking at everything like you did that goes with intake, fuel, spark, combustion. Even if your timing belt is missing teeth or beginning to, that would cause this type of problem as well, but what I think it is, is CAM TIMING especially if your timing belt is old or you had just done something relating to your head or timing of the engine. The only other thing that strikes me is if you overfilled your engine with say a 1/2 to 1 quart of oil more than it recommends on the dipstick; if you have a bad distributor O-ring than a lot of oil can and normally will run into your cap which would cause no spark to the terminals. If none of this was what the case is then deffinetly replace the main fuel relay before you spend anymore money on other external parts because it is one of the cheapest and could be a possiblity, goodluck, and let me know what you come up with.