RE-RAILING THIS TRAIN:
Go get yourself some Garnier Fructis Clear Control Shampoo. You can find it at any drug store, grocery store, or Wal-Mart for about $4-$6, depending on where you live/if it's on sale/whatever. I don't think it's residue-free, and it's definitely not all-natural, but there are some people with dreads who have used it without any problems, and I think right now, at this point, that it's important for you to get your scalp problem under control, first and foremost.
Being careful not to disturb your actual hair/dreads too much, SCRUB at your scalp and make sure you rinse several times to wash everything away. I'd finish up with a COLD water rinse -- as icy cold as you can stand -- because putting my head under really cold water has helped control my dandruff and itching problems.
As a college student who lives in a tiny, cramped space that I share with two other people, a lack of disposable income, and barely any time to sleep, I understand that it's not always convenient to do baking soda and apple cider vinegar rinses, especially because essential oils can get pricey (although, you don't really need essential oils and stuff in your rinses; the baking soda and apple cider vinegar are doing all the work). I would recommend, though, that you do switch to a shampoo bar or shampoo specifically formulated to be residue-free and organic/vegan or at least pretty natural as soon as it's financially feasible. I'd recommend getting one that has tea tree oil or other astringents in it, since you do have scalp issues.
If you have hard water, http://www.dreadlockshampoo.com/ is a good place to get shampoo bars that will rinse clean. I haven't used it myself (I don't have hard water, and Dr. Bronner's has been working for me), but there's a thread in this forum where people have had a lot of good things to say about it.