On ammonia: I don't know either way if ammonia can kill that far down the road, but unless there were necropsies done on the fish to confirm cause of death, I think it's unwise to blame ammonia from days, weeks, months prior.
Disease: Inverts can and will bring in contaminated water. Hermits can keep water in their shells for DAYS, and survive out of water for that long at least. The same with some snails. So if the inverts were introduced at the time of the fish, it's a good possibility disease is the issue. The fact on disease is, most of the time we cannot see, externally, whether it's present or not. Again, necropsies, microscope slides, etc are the only way to confirm and simply not feasible for most.
Now, having said that, I will say, I would not give up. First, you have a very, very challenging size tank. Small is by no means easy. If anything it increases the challenge ten fold. Personally I think you're starter tank is too small. But that's just my opinion. I'd go with a 75 gallon as a start. Or if you only want small, include a sump that increases your water volume by 1x-1.5 times. (Sorry can't recall if you said you had one.)
Live rock: Not sure what kind of live rock you have or where you acquired it, but it's not all created equal. I have a personal bias for natural live rock. Yes I'm aware of the environmental impact but man cannot re-create in hours what it took nature millions of years to form. Good live rock can make or break a tank, in my experience. I had a lot of issues until I figured this out.
Fish: I know people say fish are where one should start. Personally, I think fish are a HECK of A LOT harder than people give credit for. WAY harder than a lot of corals. With corals you need a good light and good parameters. Corals are forgiving. They start to go south you have time to correct a problem. Half dead corals will grow back. That's something fish cannot do (unless you have one of those rare alien eBay fish). You're doing well with the inverts. Inverts are not easy. The one thing they have fish don't is they are disease resistant to what kills fish. I agree with what others have suggested. I think you have something in your tank that's managed to escape detection. Let it sit fallow, do some easy corals, like leathers, gorgonians, sponges, etc. Enjoy them, then start with fish in about 3-4 months. I'm not familiar with the light you have, but an inexpensive 4 bulb T5 would probably make success with corals much easier (as well as your BTA--they need good light). I've tried expensive LEDs myself, and I've not had good luck with them. When I went to cheap, simple T5 my corals took off like gang busters. I'll stick with those, although LEDs are pretty for the color glow.
I hope you don't give up. I mean, if you love the prospect of saltwater, i think you're cutting yourself short. We all fail at some point, at something. Failure is hard but it makes success sweet. I hope you don't throw in the towel just yet.
Take care and good luck.