[CallMeMrRef]

I do not believe the secondary defender rule relies upon the point of contact, rather it begins with initial legal guarding position.

The rule states:
Art. 7. A secondary defender cannot establish initial legal guarding position
in the restricted area for the purposes of drawing a player control foul/charge
on a player who is in control of the ball (i.e., dribbling or shooting) or who
has released the ball for a pass or try for goal.

It does not state that a secondary defender cannot be in the arc at time of contact. We know that after establishing legal guarding position, defenders are allowed to move backwards, even on an airborne player.
Rule 4-17.6e. Exception: A secondary defender who has established initial legal guarding position on an airborne shooter/passer may not move laterally or obliquely to maintain legal guarding position. The secondary defender in this position may remain stationary or may move backwards.

As to the C making a charge call, IF the play is called an RA play, the RA play trumps the charge call.

A.R. 235. As A2 makes a drive to the basket, B1, a secondary defender,
establishes his initial guarding position within the restricted area. Contact
occurs. One official calls a charge while another official calls a block.
RULING: B1 was a secondary defender who illegally established
initial guarding position within the restricted area. Consequently, the
blocking call against B1 is the correct call.
(Rule 10-1.14 and 4-35)