After you get married she can get a Dependant Visa and then she is free to work, study, etc. There have be a negligible number of Dependant Visas granted for unmarried couples. I have not seen reports that don't involve children, a relationship of many years, shared property etc etc. And it takes many months to go through the process in these cases, by which time you'll be married anyway.
Before you get married, if you can show very firm marriage plans then she will probably be able to get a Long Term Visitors Visa. Soon after you arrive you should go to the Extension Section at 5/F Immigration Tower with the original documents which show the firm plans that you have to get married (venue reservations, invitations, etc etc).
If it's early next year that you plan to get married then your fiancée will very probably simply be able to get the 2 or 3 visitor's visas that she needs by going to Macau or elsewhere. You could simply plan to do this and only go to talk to Immigration about an extension if they start questioning her.