Taiwan --> HK --> SH --> HK --> Taiwan

If I book a return ticket from Kaohsiung to Hong Kong, then a return from Hong Kong to Shanghai, I guess this does not qualify as both the origin and return destination is Hong Kong (Taiwan is discounted)?

Is the only way I can fly from Taiwan to Shanghai using the transit visa is to have three single tickets, i,e TW to HK. HK to SH. SH to TW?

What about booking a direct return from TW to SH, and a single from SH to HK, but then I cancel and get a refund on the HK flight after I've got my free transit? Would this be picked up on departure?
Thanks.