Maybe you expect too much or maybe you just don't understand general retail practices, but when you exchange an item, you usually don't get a refund. An exchange is just that, trading a broken/defective/unwanted item for another of the same value.
I'm not aware of any retail establishment that has a general policy that both lets you exchange an item and get a refund on top of it. Refunds are usually only reserved for when you flat out return an item and take nothing in exchange.
Also, why would need a general increase in credit if your balance is zero? If it's for a single purchase you're looking to make, then let them know that at the store when you're making the purchase and it'll likely be reviewed for you on the spot.
I can't speak as to HSBC's policies but in the general world of lending, the rep you spoke with on the phone is correct in that your payment history with them means nothing. (unless it was bad) If you have a good history with them, they then go and look to all your other lines of credit (open & closed) for large balances or late pays. Any single red flag on any other account in the last 5-7 years and they'll likely decline you, no matter your current FICO. It's just the way the lending world works.