It can be but make sure you pick out one that is light in color and lighter in weight. Otherwise, it is most likely still wet or sappy inside. For the best results, let it sit in a warm place for about a week.
Can use for floating shelves
I would but isn't siding nicer and keeps the wood from weathering?
Go to store and measure it?
yes and if painted do not paint bottom unless it is breathable
yes