Bean, I still think you're ignoring pieces of the rules that clearly show the IC loses part of his IC abilities when he joins a unit. There are clear rules set up saying what he can do while in a unit. Those include 1. leaving said unit he is attached to, 2. conferring or disallowing some of the unit's rules, and 3. count as a separate unit while in close combat. It also clearly shows he DOES count as part of the unit and not as an IC as well such as when he joins a unit that has gone to ground, he immediately goes to ground as well, and he also counts as part of the unit for purposes of being locked in combat.
The precedent throughout is that once the IC joins a unit, he is considered "part of that unit" until he chooses to leave it. The is listed on page 48 in regards to movement coherency, page 48 in regards to locked in combat and falling back, page 49 in regards to being shot at, etc, etc. Since only an IC and NOT a unit can join another unit, the IC therefore HAS to leave the unit to regain his IC status to join a unit.