Obviously, there's no way anyone can say what exactly killed your fish with 100% certainty. But I don't see what's so hard to believe that you got some borderline sick fish from different stores at different times. If you think about everything these fish have been through (assuming their wild, and not tank raised), by the time they hit your fish store they've got to be at their wits end and stressed out to the max. Depending on how long they've been at your fish store, they may or may not be settled down a bit. But either way, yet another netting and transport to yet another new place with different water parameters probably puts these little guys/girls right over the edge.
Just because folks can't pin down with 100% accuracy what the culprit is, doesn't mean it's the water. Again... what exactly in the water do your friends think it is? Copper? Chlorine? ... It's my understanding that water conditioners like Prime take care of that. Think I also read that some salt mixes have stuff in there to bind up the heavy metals. Is it high TDS
in your tap?... I don't think that's harmful to fish unless it's some astronomically high number. I'm guessing your water is on the hard side, being from Kansas, but that shouldn't be an issue as long as you're watching your pH - which you are.
Whatever it is, I think you're right to go RO
and at least remove that variable from the whole "what's killing my fish" equation. I'd just restock slow though. From your list of deaths, it sounds like you had 4 fish die over the course of less than a month. Maybe just try adding one fish at a time, with about a month in between fishes.
[Edit: PS... Just thought of something - have you been mixing up your saltwater for water changes 1-2 days before you use it? If you're mixing it up and immediately dumping it into your tank, that can cause you problems. How exactly are you mixing/storing your water change water?]