Sorry to hear about the loss. I don't think the handling caused the death. The acclimation surely didn't help, but I think it was likely the nitrates that did it in. I wouldn't attempt another anemone until your nitrates are consistently well under 5 ppm. Prompt treatment may help in the future.
Also, IMO people place too much stock in the mouths of anemones when assessing health. Although a closed mouth is a good sign, I believe that other factors such as the ability of the anemone to hold itself upright, inflation, "puffy pancake" appearance, and ability to attach/bury in the sand can often be more telling.