I would advise give the bank some time to sort it out. Banks are suppose to retain data for 7 years at least so you have enough time to dig into the details. If banks don't give you a satisfactory reply, then escalate to HKMA and SFC.
Generally post the lehman minibomb, most banks these days are very wary of mis-selling products. So challenges get checked through quite thoroughly. Unfortunately you are not the account owner so it is not wrong for the bank to withhold information from you until you obtain proper legal authorisation for representation.
Remember, the 'threat' of reputation is meaningful until you really spill the beans to the press. Once you spill it you are not going to get back any money. Keeping it now until the bank totally rejects your claim, would give you more leverages.
Over the years I have claimed back quite a fair bit of money from banks.. both in HK and Singapore. One for myself, one for my parent in law, and one for someone that came to me for help. Similar strategy works. Write, state your case, threaten with basis, and give them time to sort it out. Once I was rejected, but just write in again told them why they are wrong and what I would do if they don't correct the problem, and they actually paid me back those losses.
Patience is key and being rush wouldn't help in such cases.