the what if is simple. I'm very sure deep inside you have the answer, but do not want to speak it out. I can tell you, come 1997 if HK is still under Japan rule, it will not return HK to China. There was no treaty signed between Japan and China, or Japan and Britian on the return in 1997. Japan will keep it's conquest of HK.
Tiongs don't even dare demand Russia to return their land, and Japan keeping HK means it was still a military power, even more the tiongs won't dare to speak on negotiations, for the fear of another Jap invasion and losing even more land.