Some of the corrosion products usually adhere pretty tightly to the base metal: i.e. Aluminum Oxide to Aluminum, and Iron Oxide to Iron. They do not cross-adhere.
Increased volume is the major effect, but the surfaces become less smooth and there is an increase in friction.
The third question is an ambiguous one. The penetrant does not chemically break down the corrosion products, but it does promote a physical breakdown.
With steel in contact to aluminum, the aluminum will preferentially corrode, acting as a sacrificial anode for the steel.
Right. In that case, is the aluminum corrosion product aluminum oxide? That's pretty hard stuff, or at least the particles of it are....