You should not follow the P90x diet. that is made for adults (at least those over 18) that are trying to lose wieght. you are still growing and have no reason whatsoever to limit your food intake....
None of those things are important. The important things are a proper program for your goals/level and a proper diet. Nice supplements and lots of time in the gym add to proper training and diet, but...
You should squat as low as you can without rounding your back. I'm not 100% sure but im pretty sure the Ripp doesn't say "don't do ATG squats if you physically can" in his book.
I always feel it is rude for people to use up more than one piece of equipment at a time when the gym is busy (after all, you can only do one thing at a time). when it isn't busy, do what you want.
As you read, the first workout you just keep increasing wieght until the bar speed slows then finish off those as your work sets, so you don't have to...
That chart does not say when you will get stuck, it shows how "good" your lifts are compared to others of the same weight. I'd give it a couple more tries, then deload if you can't break through it.