My advice is always to expect nothing and be surprised if you get something. I think your SIL will go ga-ga for the slipper, and since it's not super-expensive, it'll be worth it even if the restaurant happens to give them a free cupcake or something.
They should be able to find clip art of any Disney character, but I'd be prepared with images to send them just so you get exactly what you want.
Garden Grill is serviced by Epcot's bakery, which can do only basic 6"/8"/10" round cakes. I'd talk to the Yacht Club Bakery about whether they can deliver a mini cake to GG for you.
Sure! I think having Rapunzel's braid wrap around the tiers might be cute. Just know that the more elaborate the decor, the more expensive it is.
Nope!
Hope this helps!