Dave wrote:
You don't reevaluate what units are directly involved (rolled a CC/FF die) round to round.
Dave, you have to reevaluate which units are involved/directly involved each round after all countercharges are complete.  In the most extreme example, it's possible for a second-round countercharge to base contact a new formation, which would draw that formation into the assault.  Obviously, you can't say a unit in CC is not directly involved.
As another example, consider a more typical attack with a support formation.  In the first round, the attacker only has 2 defenders in range after countercharges.  They are the only ones "directly involved."  Between the assaulting formation and the support, both defenders are killed.  After a tie and countercharges, 4 more defenders end up in range of the attacking formation and the support formation.  You would not think those 4 defenders remain immune to support fire in all subsequent rounds because they weren't directly involved in the first round, would you?