RBFing the transaction from A to B would still leave the transaction from B to C waiting if the B to C did not use enough of a fee.
RBFing from A to B would change the TXID and therefore invalidate B to C and any other unconfirmed children.
Yes, exactly. I knew this, which is why i was confused - but after reading your explanation it does make sense that C has the ability to increase the fee for both transactions without anything being invalidated.