A: Probably could as long as the diameter would fit within the square. There is nothing really structural about the footers, they are mainly decorative. The set screw should sill easily reach the round baluster.
A: This shoe has the set screw in the side and just needs to be tightened.
A: This is your choice. Typically, they are only placed on the bottom to clean up where it is inserted into the tread, kneewall, or shoerail. However, they are decorative and can be used on the top as desired.
A: The small hole on the side is for a set screw.