Is that even possible? I'm doing my daughter's birthday cake for next weekend (I'm thinking bake Friday morning, decorate Friday night/Saturday for the Sunday party - I need to allow time for all the other crap I have to do, too - how early can I get away with finishing a cake with buttercream?) and I was wanting to do a 10X4 layer with a 6x2 layer on top (for her smash cake). My 10 inch pan is a 3 inch deep pan, though (I don't even remember where we got it). Can I just put in the amount of batter for a 2 inch deep pan and bake it like that, or will that cause problems?

Baking in 3in deep pan is no problem - the layer can even "climb" a bit higher then in 2in pan.

As for decorating early - of course you can. A grocery cake will seldom be as fresh as your cake baked on Friday. To get even more time - you could bake the cake right now, freeze it and on Friday only decorate it.