Is SIN-PVG-TSN-PVG-HKG-SIN allowed? Arriving in PVG on 15 May and departing on 19 May to HKG.

Unfortunately, your both options are ineligible for 144 hour visa free transit.

As you can see, your itineraries have two mainland Chinese destinations, namely Shanghai and Tianjin in a row and that is not allowed under this option.

You can visit just one transit area and the immediate points before and after it have to be in two different countries/eligible geographical entities. For example, an itinerary:

SIN - PVG - HKG

Is eligible for 144 hour visa free transit because the mainland Chinese destination has immediate points before and after in eligible entities namely, Singapore and Hong Kong. So, if you want to visit TSN (Tianjin), you can do it in the following way:

Singapore - Shanghai - Taipei - Tianjin (or Beijing because they are both in the same transit area) - Hong Kong (or Singapore)

Like this, you will be eligible for 144 hour visa free transit twice, once in Shanghai and the second time in Beijing transit area.

Thank you. Can I go SIN-HKG-TSN-SIN?

Yes, it is eligible for the 144-hour visa-free transit.