I think part of your problem, Lasereth, is you're only looking at one number in regards to judging your weight loss--your weight. Do you have any idea what your body fat percentage is? Because if you don't, that's the number you want to know.
At this point, you've lost a significant amount of weight, and so it's time to start evaluating your body composition, as you've already started to do. But a key piece of evaluating that composition is your body fat percentage.
I really recommend getting a scale that also measures your body fat percentage via bioelectric impedence analysis. Just don't drink a ton of water before weighing yourself, or you'll throw off the result. It's not as accurate as other methods of measuring body fat percentage, but it gives you feedback more frequently and for less cost than other methods.
Yes, you might gain weight when you eat a more balanced diet, but that weight isn't always stored as fat. It might be that you're gaining muscle, and that is causing your weight gain. That's a good thing! Muscle burns more calories than adipose tissue, and will boost your metabolism. Plus, you may just be temporarily gaining water weight.
As for the diet soda thing--I switched to diet soda a year ago after drinking regular soda for years. I lost 45 lbs., and I've kept it off. I still drink a diet soda or two a day. But it's important that diet soda not be your only beverage--I also drink green tea, orange juice, and milk. I also like a cup of sugar-free cocoa in the winter. Yum. Oh, and coffee. Lots of coffee.
As for the dehydration issue--I rarely, if ever, get dehydrated, and I don't drink a whole lot of plain water. I get my water intake from the foods I eat and the other beverages I consume. I naturally drink more water on hot days or days I work out, which is as it should be. Here is an interesting article about water intake from the NYTimes:
http://query.nytimes.com/gst/fullpag...prod=permalink