I think the OP is referring to the rules of booking a package (you must book a package reservation to get a dining plan). If you book a package reservation everyone listed must have identical package components - same tickets, same dining plan, so if one party member isn't staying as long as the others they are stuck with more tickets and dining plan than they need. Only DVC does not need to buy packages, so they can add a dining plan no matter what.
There is now the option to book a ticketless package with just the room and dining, but only by phone, not online. This will help with the ticket issue. AP holders could always book a ticketless package (and they can still book them online if they book via the WDW passholder site). This will not work on the
free dining or Stay Play Dine promotions, as to qualify for those discounts you must book a package including a minimum ticket purchase (it's 2 day tickets right now for those).
Dining credits are pooled by room and not by guest - any guest registered in the room may use them (child's TS credits can be used only to purchase meals from a children's menu no matter who actually eats that meal).
As to who can buy a TIW membership, the above listing is correct and the cardholder must be at least 21 years old.
With a TIW it makes no difference who has what ticket. Once you are eligible to purchase the TIW you may apply that discount to anyone who dines with you (up to 9 additional guests). The two items to watch out for here is that according to TIW rules, all discounted meals must be on the same check, and the cardholder must pay that check.