Just because your mother is from HK and has a HKID it does not give you the right to obtain a HKID, you need to have stayed in HK for longer than 7 years, why do you live abroad and do you ever intend to settle here in HK. Why did you parents not apply for a hk id junior before you were 16 or 21?
The system changed and as Morrison said, all you need to do is enter hk for the stamp, mail the application and then fly off but TBH they really need to clamp down on this as people are getting HKID for laughs and i do not think you should be ableo to do this as it ruins the whole HKID.