I'm really sorry to hear this.
For what it's worth, I saw the same patterns on my 75g for the last 2 years. Everytime I thought I was in the clear, I'd try some new frags, and they'd never make it. Same thing. Start off with less polyp extension, then tip bleaching, then eventual RTN.
For what it's worth, I never really solved it either. I just basically embraced zeovit and hoped for the best, thinking that if I can't solve what ails them, perhaps I can make them so strong that they grow faster than they can die. Does it work? Well, .. maybe. I have still lost frags in the last couple months, same pattern. But at least it doesn't appear to be 100% losses (so far ... touch wood
), so there may be an improvement.
But embracing zeovit shouldn't be the solution. I just did it to grasp at a final straw and see how it would go.
So I hate to say it but I think whatever it is, may indeed not really be solvable. It's not the usual suspects of parameters, it's not nutrient buildup, it's not red bugs and it's not flatworms. (I think.) .. I believe it to be pathological in nature (ie., bacterial, protozoan or perhaps fungal). Whatever it is, it can remain latent in an otherwise SPS-fallow tank for long periods of time.
If this is the case, then I'm afraid I can only offer the suggestion that I think your idea of throwing in the towel (on the tank at least, not overall in the hobby) is really the only magic bullet solution. Start a new tank, new glass, new sand, new rock, new everything. Sell the rock to someone who wants it for a FOWLR (since clearly fish are not at risk, at least mine don't seem to be).
It is basically my plan. I'm planning on all new rock for my 280g, which is one of the reasons the tank is so delayed in startup. I didn't quite realize when I was planning a 280g that I'd be looking to buy all new rock when the time came to put water in it ...
Oh well, that's neither here nor there. I'm sorry to hear of your continued frustrations. I know what it's like, because I've been there.. The worst part is I'm not really sure that I'm not still there or not, I'm just hoping for the best.