Here are some of my answers to your questions, but as always, everybody is different so what works for me may not work for you. Experiment and adjust to your personal needs!
1. When training, I usually drink around 24 ounces of Gatorade for long runs (20-23 miles). That is simply because I carry one of those Gatorade bottles which happens to be 24 ounces! But if I need to, I will find a water fountain and drink more. Most people will drink a LOT more than I do. You have to experiment for yourself to see what works. During the race, I might drink about 3 to 4 ounces (half of a small dixie-brand paper cup) per water stop (every 1.5 to 2 miles usually, depending on the race).
2. I eat what I like, but over the years it's basically trended towards more and more healthier fare. While it is true that if you run a LOT, you might be able to eat almost anything you want, it still is not healthy to eat too much junky food. If you are serious about your marathoning, then the healthier you eat, the better off you will be. Look for more nutrient dense foods so you don't waste calories on non-nutritious foods. As a marathoner, you need to make sure you get all the vitamins and minerals your body will demand. You will probably need to up your calcium/vitamin D intake as your body will need it for bone remodelling due to the stress of the amount of weight-bearing exercise (running) you will be doing.
3. The best way to prevent injuries is to build up your distance and intensity slower than your body can recover from it! Don't run too far or too fast. Basically you get better when your body recovers from the training, so it's really when you're resting that you're improving. The training is breaking your body down, and the recovery is building it back better than before, but if you break yourself down faster than you rebuild, that is when you get injured. A few other things I do to avoid injury is to mix up the surfaces I run on (once or twice a week I run on the treadmill instead of pavement), I do strength training usually twice a week, and I stretch gently after running.
4. I don't do any stretching before running. Some studies have shown that static stretching before running reduces muscle strength. As for warm up, I usually walk for a few minutes and then the first mile or two will be fairly easy pace. Kenyan runners have always "let the run come to them". So at the beginning of the run they may start out at 9 to 10 minute pace per mile whereas the end of their run may be down to 4:30 to 5 minutes a mile!
5. I usually walk a few minutes after the run to cool down and then do gentle stretching.
6. Off days, what's that?

Seriously, I only take one day off from running, and usually that is one of the days I will do a strength training session. If I'm feeling beat up, then I may take a day off because it pays to listen to my body.
7. Everybody's running schedule is different. But one rule that is universal is to avoid too many stressful days in a row. Many runners use an hard/easy varying schedule. That means one day they will run a hard workout but the next day they will recover with an easy workout. The easy day means you still get in the benefit of running without too much additional stress. Some people may vary it by putting 2 hard days together, but then they will make sure to take 1 or 2 easy days after the block of hard days. Anyhow, since you asked, here is one example week of training for me:
Monday - 10-12 miles easy
Tuesday - 7 miles very easy with a group, strength training
Wednesday - 15 miles, 2 miles easy then 13 miles at marathon pace
Thursday - 10-12 miles easy
Friday - off
Saturday - 12-14 miles with a group at varying pace, mostly easy to very easy, strength training
Sunday - 20-23 miles ranging from 20% to 10% slower than marathon pace
Anyhow, I dunno how much this will help you specifically... but good luck and enjoy the training and the marathon!