Tangs, especially of similar shape, can be very dangerous to one another, although your tank is large enough that this shouldn't have been a huge problem. Of course, when fish die, you very seldom will find a huge rotting corpse laying around, because their instinct is to go find a hole and die in it, just like other land-based mammals. In any case, I've had brittle starfish pick a skeleton clean in about 6 hours, and I'm talking my tangs that I've had die (we're going way back to 2007 for this experience), so that could account for the disappearance as well.
Fish eating for one week isn't much of a sign of anything. I'd rather see them eat than not, but injuries caused by cyanide and other dangerous (to the specimen) collection techniques often will not manifest themselves immediately and may have no bearing on the eating habits of the individual. Copperband Butterflies are notoriously difficult to keep alive even when eating prepared foods due to this.
Ultimately, some individuals just waste away slowly, even if eating. There is simply no way that we can come close to offering the variety of foods that these animals are offered in the wild, and some of them just do not adjust. Cyanide exacerbates this process, but is by no means required for it to occur. Failure to thrive and ultimate wasting away of an individual can take days, weeks, or months in our aquariums depending on where else they have been before making it to our tanks.