Before I commit to purchasing tickets, is this route valid for 144h TWOV?

LHR-PVG-HKG-PVG-LHR?

I am looking to purchase LHR-PVG-LHR with BA and then PVG-HKG-PVG with a different airline (whichever is cheapest). I am travelling with children and we are all British Passport holders.

Also, on the return legs, I presume if I travel on the same day, I would still need to complete 24h TWOV form and collect baggage and then check-in for second flight; is this correct?

One more question: Does the online form work or is it just easier to fill in forms on arrival?

Sorry for so many questions. Thanks.