Yes. Yes. Yes.
Any shop owned by Disney will take them- there are a few in the parks that are not owned by them. Downtown Disney is mostly other vendors- restaurants and shops so not there except Disney's stores. Some people have bough their tickets with them- I think you have to call and not do it online because you can only use so many gift cards at a time online. That is where you are saving big bucks because 5% on a $50 gift card is just $2.50. Not much of a savings- but every little bit helps.