Well, I can't answer your specific questions, but perhaps our hippo tang experience can help:
We purchased one a couple motnhs ago (our tank's been operational since January) and came back to our LFS in a fluster because the poor guy was riddle with white (spots and/or cloudy areas - kept changing). LFS advice was: Hippo tangs are sensitive creatures and particularly prone to parasites. Parasites are a natural part of a reef system. You can try to kill off the parasites, with some chemical additions which won't harm the corals, but it's expensive and only temporary. His advice was to see if the guy can tough it out - and either he will or he won't. While I was there, another customer related his tale of a hippo that was almost completely white when he took him in and he lived for years.
So, home I went with a $20 bottle of Chem-Marin "STOP" for parasites just so I felt like I was armed with something. I haven't ever opened the bottle. When I got home, he looked better already. He's been doing fine for months - will periodically get white patches, but they eventually go away, so I assume he's toughed it out. We also have cleaner shrimp, which help him out a lot and he rubs himself against the rocks all the time (he's also taken to rubbing himself on our mushroom coral - hasn't seemed to bother either one of them, but it's kind of funny). I worried about him getting scratched (possibly that's what the black lines are on your's), but as I say, he's been just fine.
I'm not aware of peppermint shrimp being cleaners. Give the cleaner shrimp a chance - they have to establish a spot (cleaning station). I also understand that the cleaner shrimp are somewhat social - you may want to have at least two in the tank.
Hope that helps.
