A unit can be activated twice per turn. Once to move and once to fire. Either way, the unit has been activated. If so, they aren't very effective against these targets. Once in the assault hex, the units may conduct assault whether a leader is present or not.
Units move individually.
Assault is resolved when a unit moves into an enemy occupied hex. Assault combat is resolved immediately after all units activating on the same action segment and directed by the same leader enter the assault hex. So no, you don't resolve assaults one assaulting unit at a time. I know one attack can be up to three units. I was a little puzzled about the wording in 8.2.