I will make a strong suggestion that can be a tremendous help for future additions to the tank. First, let me say that as I read this thread I find it impossible to eliminate or reduce the possible causes. As I brainstorm on the possible issues, I come up with a ton of things that "could" be a culprit. Initial capture, shipping, acclimation, health of fish at the LFS, pH swings, CALCIUM LEVELS, alkalinity, design of live rock structure, behavior of other fish (especially the Lemon Peel), diet at the LFS compared to current diet, etc etc.
There are just so many considerations that it is difficult to help quickly. My suggestion is this, and I realize that hind sight is 20/20. However, the use of a quarantine tank would eliminate many of these issues. A quarantine tank can be set up quickly and inexpensively, generally for less than the cost of 1 or 2 fish, especially if you have some used equipment lying around from your likely days as a freshwater hobbyist.
There is a nice article in our library on setting up quarantine tanks: