Sorry to hear about all of your losses - live and learn i suppose
I agree with dark, your tank is still cycling. Not to say that 2 years ago it wasn't cycled, but after adding all of that livestock, then having all of that livestock die, i think you are in a cycle. Leave the tank alone. It sounds like you have sufficient rock and sand, so once the bacteria is populated your levels will even out (except nitrates, water changes will take care of those). I think you've had high Ammonia this entire time because you keep doing one of two things 1) adding more things to the tank, fish, rock, corals, etc - this adds to the bioload and will product ammonia, especially in a tank that doesn't have enough bacteria to break it down to nitrite and nitrate. 2) you keep doing water changes, so your bacteria isn't able to populate, leave the tank alone. If your ammonia reaches around 1.0 then do a small water change (10%) to drop it a little, but you want ammonia in the tank so that the cycle can complete. Once you have 0 ammonia and 0 nitrite, you will have a good amount of nitrate, do a water change at that point and you are ready for fish. ONLY ADD A FEW AT A TIME, i'm sure you've already learned that, but i just want to remind you. Only add one or two, wait a few weeks, then you can add a few more.
I think the issue is you are trying to get rid of ammonia and nitrite, two things that will go away themselves once the tank is properly cycled. You trying to get rid of them will only prolong the cycle process (which cycling is a good thing).
Also, check around to make sure you don't have any more dead critters in the tank, that will also produce ammonia.
