On the corals that were in trouble, I'd lean toward checking and rechecking alkalinity. However, nitrate of 30ppm can be part of the problem, that's for sure. Some corals can't tolerate more than 1ppm (I've read), but many seem to adapt to 20ppm or less. In my own tank, some corals went up in smoke very quickly while others have been with me for years and years, through the giant spikes of phosphate and the ongoing higher nitrate. Both are much lower now, and I still have those same corals.
Sometimes you just have to accept that certain corals will never do well in your tank while others will thrive no matter what. It never hurts to try a coral again, if you feel so inclined. I've tried several blue torts over the years, and somehow I was able to keep two small pieces alive for all of 2008. They aren't much to look at, but they are alive.
That new coral is an A. millepora - and a very pretty one. The tips as they grow will be lighter, but not bone white. I think you are seeing new growth.