Can you put private keys directly into Coinbase or Coinbase Wallet?
Not your keys, not your bitcoins.
You already have a cold wallet, and let's think why you decided to use that cold wallet? To keep your private keys offline, secretly. And now you want to import it to Coinbase wallet?
It is conflicting activities.
Just don't.
he needs to sell his btc.
at op if you are usa based coinbase cash - usd is fdic insured.
I do not know how much you are cashing , but lets pretend it is 15,000 usd worth of coin.
send 1,000 once it hits sell it keep it in usd - cash not any coin of any kind.
that is now FDIC insured (assuming you are USA based).
so do this 15 times 1000 each time. you never risk a large percentage of the coin.