Cavalry Charges

[Hugmenot]

12.2 Required Combat
Assault combat must be resolved immediately when active units enter a hex containing enemy units and no friendly units. Resolve the assault after all active units which are to enter the assault hex this action segment have finished entering the hex.
All units of both sides in the hex must participate.

My understanding of Rule 12.2 is units entering an assault hex do so one at a time and thus your Case A is correct.