I've never baked these particular cupcakes, so I can't say much for the oil subs. Maybe make a test batch before hand?
But if the recipe yields 12 cupcakes, it should convert over to 8 inch pans pretty well. By "sandwich cake" do you mean a basic layer cake, or like the bread-slice pan? I am usually able to switch a cupcake recipe to one 8-inch round/square and simply adjust the time for the smaller amount of surface area. 32-35 minutes should do the trick.
Isa applies the same logic to the VCTOTW/IDI chocolate cake recipe: http://www.theppk.com/2008/08/just-chocolate-cake/
It's exactly the same in both recipes, but one is for an 8-inch round and one for a dozen cupcakes. If you're doing two layers, definitely double. You'll probably want to double the frosting, too. Good luck!