China as a base

Hello! I am planning to get a return ticket from London to Shangai for 3 weeks. We won't be staying in Shanghai for this long as we plan to go to South Korea and Japan within this time, it just works out cheaper for a return as opposed to individual one ways. So the plan is:

-London to Shanghai (spend one night)
-next day fly to Japan (10 days or so)
-Japan to South Korea (approximately a week)
-South Korea to (re enter) Shanghai, however only spend 4 days here and then fly back to London

I guess my questions are:

1) are we still eligible for TWOV 144 hours considering we are re-entering but not spending the fill amount of days here?

2) if not, are we entitled to only one TWOV at a time? could we use two within a few weeks?

Thanks so much for your help, finding official guidance on this is proving very difficult.