London-Hong Kong-Tianjin-Hong Kong-London. Is this ok?

Planning on going to China next year. Plan to be in Tianjin for 4 days. My question is:
A) Is this route acceptable for obtaining the 144hr visa-free transit (London-Hong Kong-Tianjin-Hong Kong-London)
B) Would the Hong Kong flight need to be a return flight or can it be two singles?

Many thanks in advance.