Thank you all for your advise and please keep any suggestions coming as it is still not resolved.
Honestly 6 days is a long period for a pending transaction (the transaction was not processed yet because as you said, there is no transaction hash at the receiving address). Although it may happened, there must be a transaction hash on your Coinbase wallet even it was pending but unfortunately, none. And you are 100% sure that everything is correct.
Coinbase is the one who can help you here since it wasn't process yet at their side, no record at the blockchain yet. Fees is also not the issue as Coinbase is using above the average fees for every transaction. The best thing you can do is to send Coinbase tickets "everyday" so that it can always bumped. I said "everyday" because it's alarming and I understand your frustration.
Hope one of these days Coinbase will respond to you to see what errors happened during the transaction.
At Bittrex, if you start a withdrawal, the first status of the withdrawal is 'pending' and if you confirm at the link in the email message, it changes to 'Authorized' and a bit later it shows up in the blockchain and in the wallet with 0 confirmation first, and later on it gets confirmed.
Coinbase doesn't have a confirm withdrawal email.