So, what, exactly, is a turn?

[S'mon]

While you can react in between squares moved per RAW, I would only allow a Readied action to interrupt a charger's attack once move is completed. I would not allow a Delayed action to interrupt in between the move & the attack.

Ergo:
Monster can move to flank, delays, friend charges flank & attacks with CA, monster attacks with CA

is ok. Monsters can't charge the PC from 2 flanks and both get CA though; only the second one in gets CA.