Re: The week before..
1) Yes, there will definitely be some main draw players there on Thursday and Friday -- potentially anyone who's not at the New Haven tournament. I've seen players around practicing as early as Tuesday/Wednesday.
2) The West practice courts (P1 through P5) are where I've usually seen the big names. That's where Sharapova is in the second picture. Unfortunately, those courts are behind a fence, so I don't think it's the greatest place to hang out unless you're just hoping to get autographs from players on their way out. It used to be possible to get a decent look from the stands on Court 4, but last year they put up an extra railing with a big tarp on it to block the view. (Ahh, the USTA, so fan friendly!)
Hopefully someone else can answer your question about the show courts. I've heard about players practicing on them during that week, but I've always been too busy watching the qualifying to go see for myself. You could also try the East practice courts, which are just outside the main entrance (near the Grandstand). I think it's primarily lower ranked players who use those courts, but Dementieva prefers them, so you never know.
3) The qualies are scheduled on almost every outer court: 4 through 17. On rare occasions (like if the schedule gets backed up by rain), they'll also play qualifying matches on the Grandstand.