M4:
Mike Mentzer and the rest of his small following of H.I.T. squad believe in the one all-out set to complete muscular failure for a muscle group is all you need for muscle hypertrophy. This is not only until you can no longer complete another positive rep, but you keep going with the assistance of a partner doing negatives and partial reps. Once you complety work the muscle like this any more sets are not only not necessory but counter-productive because they they create inroads into your recovery, thus leading you to a state of overtraining.
Now, I must tell you that I do not believe in this this type of training. Even Yates doesn't train like this. He will do two or three progressivly heaver weights until he reaches his max weight, then train to failure; however, while he is often thought of as a one-set-to-failure trainee, he will do three or four total excerises per muscle group - not one. So even he ends up doing 5-8 total sets per bodypart.
There is a lot of good info on the net about training, but there is also a lot of crap. Knowledge is power so you will need to gather as much as you can and try to put it together. So to answer you question, Q1: yes multiple sets per excerise is much better than one for strength and hypertrophy. Also two or three sets per excersize per bodypart are also best.
Q2: You do want to do multiple excerises per muscle group as they will work the muscle differently.
Q3: Eat good and you shouldn't need much except for a good multivit and a protein drink if you don't expect to eat for a while between meals.
Also, don't forget to give your self plenty of rest in the form of sleep, between workouts, and between bodyparts. The only time I train on consecutive days is when I'm dieting, other than that, I have at least one day between workouts and split my workouts centered around the squat, bench, or deadlift. Everything else you do can be grouped with one of those lifts (shoulders and triceps with chest, abs and calves with legs, and upper back, traps, and biceps with the rest of your back).
WOW, what a long post!