mine is premixed so don't need to mix it unless its really old and has been sitting for along time.
Yes. However, you should clean glossy surfaces with sandpaper or TSP before applying.
Setting time varies depending on the temperature and the humidity. For sanding or adding a second coat, I would recommend waiting at least an hour. More would be better. For painting, I would let it sit over night.
Not flammable, but I would not recommend using it for mortar in a fire-pit...
Yes you "can" and about 95 Sq ft
No, you need something to be taped across hole, Then use joint compound.
NO need. It is ready mix.