Well I raised show goats for FFA, and I should definitely be able to help you with my knowledge. :)
Question #1: I fed my goats Honors Intimidator. My big Houston goat was so spoiled, I fed him the Lindner's goat feed, which was $20 a bag. The other stuff I fed was around $16 a bag. You're basically paying for protein. I'd feed a feed with a min. Of 16% protein. My feed was always pelleted, I didn't even know they made a sweet feed for goats.
Question #2: My goats ate 3 lbs per 100 lb body weight a day. If they weighed 30 pounds, 1 lb, 65 lbs, 2 lbs, and if they weighed 100 lbs, 3 lbs of feed a day. You could feed it all at once, or break it into a couple different feedings, such as if she weighed 100 pounds, she'd get 1 1/2 pounds in the morning, and the same in the evening.
Question #3: My breeder fed his mama goats hay, but I fed mine alfalfa. They were given a nice handful 2-3 times a week. A handful in the morning, and a handful in the evening. I always worked with them every day in the run for atleast an hour, so they got an hour of grazing time in each day as well.
Question #4: I wormed with the SafeGuard goat wormer, tried every 28 days. I have a bad memory though, so sometimes they'd get it later than they should have. Mine were wethers, so I'm not sure of the worming for dairy goats. I know my breeder did worm his mamas with ivermectin and another wormer, even while they were nursing so I'm not sure if it's such a big deal.
Question #5: Kids seem to prefer one teat over the other, so that may be why the milking is off-balanced. Signs of mastitis are swelling, discoloration, or any lumps or anything on the udder. Check the milk to see if it changes color or texture/thickness, and the taste. Low-grade mastitis may be present if the milk tastes sour. Here's a website that may be able to help you out some more in case you are worried. To me, right now, it looks like she has no issue. MASTITIS
Question #6: Dairy animals of any breed, especially cows, always look emaciated compared to the meat breeds. Take the Jersey cow for example. I personally think they look disgusting, as if they're about to fall over and die from starvation. It's just how their body develops, and is actually desired in dairy conformation judging. I never had a problem with my goats gaining weight, but then again I raised the boer goats that is the most desired goat for meat because of their weight threshold. With a steady diet of pellets and alfalfa/hay, and free-range grass, I wouldn't worry about her looking thin. That's just how some breeds are, really nothing we can do to change it.
A nubian dairy goat: http://www.pruittvillefarms.com/KATIEANDHONEYSUCKLE.jpg
Compared to a Boer doe: http://www.childersshowgoats.com/Siennastarweb2.jpg
As for the collar, I always use a dog collar. On goats we used collars, and on lambs we used rope halter just because they were so tall. You can either use a dog collar, or a show collar, which is metal with a piece of plastic tubing to hold on to. Your local feed store may have them in stock for the show kids. http://www.scottdalesupplyonline.com/Catalog/GC20_L.jpg
Hopefully I covered everything you needed to know! If you have any other questions, I'll help you out as much as I can :)