Since you mentioned it, in this scenario there is a big possibility that the Visayas and Mindanao regions will break away from the PH and declare their independence. Its hard to predict if the leadership of these two new entities will be friendly, neutral, or hostile to China.
Mindanao definitely, it just need a spark BUT frankly speaking the Dutertes are the one that is impeding the secession movement cause the family still believed in the Philippine nationhood movement, maybe a war with China is the spark as the locals feel they were being marginalized under the Marcos administration.
The Visayas since it's composed of many Island only Cebu, Negros and Panay will secede, while Leyte will stay because of the Romauldez influence in the Region.
We had a precedent, during WW2, the Japanese only invade the Luzon proper and when Manila Fall, the resistance from the province collapse with it, event though the struggle in Bataan is still ongoing. The reason is basic, Imperial Manila hold the economic and political thread that binds the country together. Just look at where the former US bases were located (Subic at Zambales and Clark at Pampanga) and why they insist on getting it back. And also Luzon hold the most develop region as most of the critical infrastructure like major ports, power plant and the most important oil depot are located, it's a major distribution HUB for the rest of the country.
