Actually, free weights are perfectly acceptable in the book. Some HITers prefer machines (doesn't have to be Nautilus) because you can set a weight very quickly and you don't have to rerack the weight (as most gyms require), so you can head for the next exercise in your workout with minimal rest.
Because you can't get a total body workout with only one exercise. It is actually 12 exercises for beginners and as few as 8 for more advanced lifters. It's not that much different from a HVT split that uses 4 exercises for 3 sets each, for example.
Why take multiple sets to failure, essentially beating a dead horse, when you can just take one set to failure? Same results, one set does it in less time.
The more advanced you are, the less exercise you need to be sufficiently trained.
Because the speed removes a lot of tension from your muscles. You only have one set, so you want to actually work your muscles rather than pushing big weights to make your ego happy.
What I am about to propose will sound stupid, but bear with me:
Let's say that you have a job that is 10 miles away. You have two choices of getting there: walking or driving. Which one would be faster?
There isn't a bloody study in the world that shows that driving is faster than walking to get to a place that is 10 miles away. But you drive to work anyway because you can use your brain and you don't need researchers in lab coats to tell you how to live. So why do you need researchers to tell you how to train?
I gave HIT a try more than a year ago and I haven't used another type of program since. It's not for everybody, but it is definitely for me. You'll never know unless you try it. It's just one bloody workout. If you don't like how you feel afterwards, hey, you only wasted 30-45 minutes of gym time. Get over it.
There is a better forum to answer your questions better than I can. I suggest you visit them and say hi: http://www.drdarden.com