That shouldn't be the case. BTC purchased after the fork has no relationship to BCC. Bitcoin transactions included before block 478559 are both valid BTC and BCC transactions. Anything after that is only valid as one or the other depending on which chain it is included in.

After the fork, BTC and BCH are distinct and independent. So buying BTC after the fork would gives the buyer zero BCH.

So who owns the correspondent BCH (assuming that the seller was a coinbase user before the fork)? Coinbase? The previous owner?

Coinbase has the keys for them, they have stated that if they ever accessed those coins they would make them available to the wallet holders.