Sorry to hear what you are going through. I'm glad that you posted here, because a week ago you were worried that you might be dealing with a backyard breeder. Well.... ok, no need to say much more, we all get the picture.
The thing is, cats don't normally drink that much water. They get most of their fluids through their food. I've never had any luck getting mine to drink on demand either. In fact just this morning I saw Serafina messing around on the sink, and realized that she might be thirsty as I had put their water dishes in the washing machine and needed to refill them. So I refilled them with cold, purified water, two big bowls, put that out for them. Serafina walked up to it, sniffed, then walked away. Grrrr.
Anyhow, I think the tact that I'd take in your situation, is to try to feed already very moist canned food (like weruva), and add some water to it. Say, double the water content. Or maybe take a can of tuna (make sure it's packed in water not oil) and pour the water into a dish, and add back some of the tuna into it. Something like that.
Anyhow, I think you're doing the right thing, getting her help and trying to weather the storm. Read up on TF:viewtopic.php?f=9&t=3876
I think there's a really good chance that this is what you're dealing with. Oh, and btw, I'd never suggest to someone that the solution to having a sick kitten is to get their money back. That's just a copout. The kitten is not like an ill-fitted sweater that you return on a whim. So good for you for taking this seriously and stepping up as her guardian now in her time of need.