1. I'm worried about the human hair drying out my hair when it's in. Is this a real problem? And if it is, what do you do to combat it? The human hair isn't going to be in your hair. The synthetic braiding hair will. You should get some type of spray on moisturizer or braid spray to keep your hair moisturized while it's in the cornrows. If you keep your hair moisturized, it will not dry out.
2. I want it tight enough to keep in for awhile, won't that ruin my edges and my kitchen? Tight enough to keep it in for a while is standard for a weave. If your braids are not holding up when you leave out of there, I would go back and have them fix it. If a weave is done properly, it should not be too tight. It should not pull on your own hair or edges. If it feels too tight, do not hesitate to tell the braider it's too tight.
3. Finally, How do I DC my real hair underneath and also wash the weave without ruining it? Is that even possible? You can wash and DC the weave just like you do your own hair. I use the same products and everything. If you buy a decent quality hair, you shouldn't have a problem washing it and keeping it looking nice. Some ladies here on the board have suggested getting a bottle with a nozzle to get down in between the tracks and braids when washing but I've never done it that way. It wasn't necessary for me. HTH...
Any and all suggestions/input/concerns/advice is welcome.