This is one of those situations where the rules may be a little rough, but ultimately it is left down to common sense. So I would suggest that once a ZoC is entered, the attacker moves towards that unit until it is stopped (possibly by a second enemy unit):
- If the Valk is in front of the infantry, the 1st attacker moves into B-B with it (removing the ZoC even though it rises up above the melee) and subsequent attackers move as they want into B-B with the rearward infantry units.
- If the infantry units are placed in front of the Valkyries, when an attacking unit enters a ZoC (the Valk), it then moves towards that unit, but stops prematurely when it hits an infantry unit. The next attacker stops when it hits the second infantry unit, and the third attacker can then move into B-B with the Valk finally removing its ZoC (assuming it has the movement range of course).