1) There are several alternatives for studying some kind of TEFL qualification in Hong Kong, but by far the best recognised is the Trinity CertTESOL at English for Asia. This is readily accepted in a whole host of countries around the world, including by the EDB in Hong Kong. Yes, it's about twice the price of the same course in the UK. If you want the Trinity qualification and can't afford to spend a month in a cheaper country, however, then there really isn't any alternative.
2) It is not 100% necessary. Without a degree, however, and relying upon your native ability alone, the opportunities open to you in Hong Kong will be seriously limited. In fact, I might venture to suggest that your lack of degree poses an even greater barrier to your finding a teaching position than your lack of TEFL certificate. If you have the right to work in Hong Kong (i.e. don't require visa sponsorship), then job openings may present themselves. Otherwise your chances are about as good as a snowball's chance in heck.