I can't answer all of your questions, but the one about PI, I can try.
The last time that I was at DTD/PI (March "04) from 7p to 9p you are able to get into the area to shop, eat, drink and just walk around with out paying for a ticket or having to show your ticket. BUT, you can not get into the clubs.
If you want to get into the clubs, you have to pay for a ticket, or have a ticket that encludes PI. And yes, they will now if you have a valid ticket for entry at the clubs, because when you swipe your ticket (and show your ID) at the gate to get in you also get a wrist band, that lets the clubs know if you are either under 21 or over 21, you can not get into any of the clubs at PI with out the wrist band. (Or I think its 21 and over get a wrist band and 20 and under get a hand stamp, not 100% sure about the hand stamp.)
The only places that I go in PI is the Adventures Club and Comedy Warehouse. At those clubs there is no real age requirement, but as the night goes on, the comedy inside can get a little suggestive. So if you are thinking about taking young ones I would do it just as the clubs open up.
Now according to Birnabaum's WDW guide book, guest 18-20 can get into all the clubs with no problems. But you can not get into Mannequins and the BET Sound Stage if you are under the age of 21).
I hope this will be of some help. I am sure some one will come along with more info.