1) How cold is it over there?
I run open SBB, year round. If you have received your package of bees, it should be warm enough to have the bottom open, but it is unlikely that you'll have a serious mite problem right now. In general, the mite do a number on the colonies in the second year. Leave the solid board under the hive for now, when it gets warmer change it.
2) Most of the confectioner's sugar does have some kind of an anti caking agent or cornstarch in it.
The bees don't actually eat the sugar, it mostly falls right through the frames.
Some of it the bees do eat some, I guess, so you have to be careful at the end of the fall that you don't wait too late and that they'll have a bit of time (fair weather) before the bad weather sets in so that they have time to defecate outside.
The powdered sugar make the mites lose their grip on the bees, it also make all the bees groom at the same time, further knocking some of the mites off. The screen let the sugar and mite fall right out.