We do that, actually, in the hospital for people with digestive issues like colitis or bowel obstructions that can't be resolved quickly. It's an inexact science requiring constant monitoring (lots of blood draws, needle sticks) and a pretty high risk of infection at the IV site (nutrient rich solution provides an excellent medium for bacterial growth). Also, the solution is pretty caustic to the veins going in, and can cause phlebitis and other localized reactions. So, not recommended outside of life-threatening conditions.
As for #2, SEVERE dehydration can lead to kidney failure (poor perfusion to the kidneys can cause them to die), but Bird is right - the body is designed to maintain fluid balance despite fluctuations in oral intake. Think of your GI tract as a holding tank, you fill it up, then your body takes what it needs as it needs it, and flushes what it doesn't need out the kidneys. You don't actually go through cycles of overhydration/dehydration just because the levels in your tank go up or down. You just need to remember to fill it up occasionally.
On a side note, alcohol depresses the part of your brain that tells your kidneys to conserve water, which is why the floodgates open and you have to pee so much when you drink, and wake up hungover with cottonmouth. The more beer you drink, the thirstier you get, so you drink more beer, which actually dehydrates you even more. That's why they recommend balancing each alcoholic drink with a glass of water, and drinking a huge glass of water before going to bed. Keep that tank full, your body will thank you in the morning.
