Regal tang are shy and sensitive, even when I had mine it alway shy away behind the rock until you move away. Same as feeding unless it really hungry it well make a quick dash and grab as much food before it hide back. If you are near the tank, keep still and let it come out and when it know u are there and u are still it will build it confidence. But no matter how long u have it, it will still dart back behind the rock. As for chunky that is depend on how much you food it. If it still rounded don't worry about it, it is fine and if it fed twice a day and eat well that is perfect. As for colouring, it depend on it diets, varied it meal, marine flakes,algae flake and meaty food which i know I've said it many time chicken. They say tang are vegitarian eater but I don't believe it, They get a bit of everything and they're greedy eater same as the yellow tang.
Looking at the photo the regal is healthy and as for the nose patches, that can come from scratching along the rock when it dive in, not an illness. The same said for the humbugs, I've had them before it got return to the lfs, and sometime their colour was bright and sharp other time dull greyest like u say, and I put it down to their diets. Change it regularly, not the same for weeks and u notice their colouring come back to it best. Cyclopeeze is another food that is for coral and fish and it bring colour into them. HTH.
the regal tang which I love and done well for 5 years, my mate went and swapped it to our lfs for a baby regal tang because it went berserk when he bought a bluethroat trigger fish and kept attacking it. So he decided to keep the trigger fish and exchange the regal for a baby one but the baby regal dies a few days later because it was being bully by the yellow tang. But it his tank and I'm only teaching him what right and wrong.
So how is the main tank coming along now will you put the remaining fish in soon.
Tony