Scott, the Planet Hollywood voucher is for 1 person - or at least it's supposed to be. They are trying to lure people to eat there by offering a free meal for 1 person. It's a limited menu as well. The merchandise voucher has been reported to be a keychain. It's another way to get people into their gift shop to look around =)
As far as reservations, many of the buffets are booked even during slow season so I would expect them to be pretty full during
free dining, too. The character ones will def be pretty booked.
Almost all reservations do not require a deposit so if you don't show, there is no penalty. Now, it's nice to call and cancel if you can't make it so someone else can get your reservation but they won't hunt you down if you don't. You can even stop by Guest Services once in a park and cancel there if things change, or call from any WDW phone. The deluxe hotels, I know, have a guest services desk in the lobby and you can stop by and make reservations with that person.
We have run late for reservations and it's never been a problem, we just get seated as soon as they can get us in. Disney does reservations strange in that you will get the next table ready for your party size. If you don't show, they just hop to the next person on the list. If you show up late, they will be able to tell you whether or not they can work your party in at that point. There is no guarantee that 2 hours later they can but you're welcome to stop by and see. And with that, if you have a 6pm slot there is nothing to guarantee you'll be seated at 6pm. They may be running behind and you'll have a wait...
HTH!
