The only thing I am aware of is a reverse osmosis unit. They're very inefficient as you already mention. Other than that? No idea. But, after reading your post, I seriously doubt that the water quality is the reason for the unexpected deaths.
Species that are - on paper - friendly and peaceful, can still become agressive if the circumstances are right (or rather: wrong). A 'pop-eye' infection is, for example, very often a symptom for stress caused by ongoing agression. I've seen it in my tanks with males that were chased around all the time by other, more dominant, males.
Unexplained deaths cóuld be explained if the fish was chased and hit something (a rock) while trying to get away from another fish. Not unusual, especially with Mbuna! I'm not saying this is the case, but when I read your post, I think everything was already done that could be done to make the water 'fish-friendly', and water is just one of the many factors that make up a well balanced tank. That's why I thought of the behaviour in the first place, rather than the water quality.
Having said that, maybe you do too much to the water? Chlorine removal? Yes, but only if there is chlorine in the water to start with. Cichlid salt? Why? What does it do (proven effect, I mean, not what is on the packaging!)? Cichlid minerals? Same question. No need to, assuming you feed them properly. Dry Argonite? Sounds like something from Superman IV. Does it do anything to the water?
I don't know what's in the tapwater where you live, I'm just wondering if you're not fiddling with the water too much, and by doing so you do more damage than you would want to (for all the right reasons!). Less is more, in my opinion, but like I said, I have no idea what sort of chemicals can be found in your tapwater. I guess I'm lucky enough to live in a country where the tapwater is drinking water - I simply hang a garden hose in the tank and open the faucet for a water change