Is it possible to double this recipe?

Should be no problem, this is really a quite forgiving recipe; I've done half batches, adjusted the amount of juice etc. As Nancy points out, the one place where you could go wrong is in achieving the proper slushy state to stir it, without having any of it frozen solid. But if you use a fairly shallow pan and keep an eye on it (which you have to do anyway, as time will vary considerably for different freezers) it shouldn't be a problem- it doesn't have to be all that exact.

Probably yes.
But stuff in time and freezing time will be longer.
Or just make and freeze two regular size of the recipe.