Tony, I wonder if there is an answer. Most of these threads and there seems to be many, seldom have a conclusion, when a seasoned aquarist like yourself is involved. Of course, many newer ones, make some visible mistakes, that we can help with but the ones like you, usually have enough knowledge to cover all the bases.
I read your thread on RC and similar answers over there, as on all the similar threads on sps problems like yours on the RC sps forum.
I wish I had an answer for you from reading most all of them over the years but dont. Just looking at them to me, looks like burn/bleaching from excessive light at the start. Not saying thats the cause but what it looks like. I cant see them receding like that from not enough light, old bulbs, or anything of that nature.
Definitely, as mentioned, abrupt alk. changes to the corals could cause it. As for the uv working, if you remember when the newest bare bottom fad [not the one we did back in the early 90,s.
], began, most of the experienced reefers promoting the system, also said using uv was a large part of it. If you check some of those large threads on RC in regards to it, most had mention of them.
I cant personally say if they help, as its been 20 yrs. since I used one, but the evidence from them and others like Sam, may show some truth in it. Mind you these were high flow bare bottomed sps systems.
As for ozone, does it not remove organics, similar to carbon? Always thought that,s what it was for. So if your problem is bacterial, then the sterilizer would be the way to help. I have thought about getting a small hang on unit for my nano. The one problem with UV units, is not changing bulbs soon enough, and/or premature bulb failures with some models. Plus I thought I read once about salt reducing the light emitted and models with the wipers to be far superior. [ just trying to remember things here].