We finally bought the tickets! We are flying Cathay Pacific from Sydney to Hong Kong, then to Japan and back to Sydney via Hong Kong. One month's holiday. In the end we decided to purchase the tickets from Flight Centre as they were cheaper than the online tickets, despite the additional credit card charge.
I was still tempted to include a trip to Paris, as it would have been possible to fly Sydney to Beijing or Shanghai with Qantas, then to Europe and to Japan with Finnair and back with Qantas again. The thought of taking Mum on her first overseas trip to Europe has not entirely escaped me and we could fly onwards from Hong Kong, cutting down our time in China. Unlikely, however, as it would also increase the cost of our holiday quite substantially.