What happen if the decision was in China favour? ... would China still reject it?
The decision of the PCA is legally binding on China in accordance with UNCLOS Annex VII. As often stated because decisions of the PCA lacks an enforceable mechanism, China can simply ignore the rulings. This however will come at a reputational cost to China. I think the rulings will help to narrow the scope of some of the disputes and help to clarify some of the issues such that future country negotiations (be it bilateral or multi lateral) will be more narrowly defined.
I think the decision would be ambiguos or neutral and absolutely won't be clear and straightforward
The Philippines had submitted altogether 15 claims to the PCA. As far as possible, rulings and their implications can be summarised into four broad categories.
(a)We can be sure that there will be no determination over sovereignty and/or delimitation of boundaries. It is outside the scope of the PCA's mandate, and a key feature of the Philippines legal argument to enable its submissions to be accepted by the PCA.
(b)The legality of the nine-dash line. In my opinion, the PCA will likely rule that the nine-dash line is inconsistent with UNCLOS. The upshot of this is that it will not have any direct impact on China's sovereignty claims because the nine-dash line is not a conventional legal argument but an outer boundary of its claim. The legal basis itself has never been conclusively and specifically defined by China. The ruling might eventually have the effect of forcing China to actually outline the legal basis of its sovereignty claim. Such an outcome on China would be a natural development in any case because China cannot in perpetuity sustain a sovereignty claim based on ambiguity.
(c)Maritime features and entitlements.
I would speculate that Subi, Gaven, Hughes, Mischief Reefs and Second Thomas Shoal will be determined as LTE's that do not generate any 12 nm territorial sea of its own. In other words, there is no sovereignty claims to be had over these reefs. Additionally, Fiery Cross Reef, Johnson South Reef, Cuarteron Reef, and Scarborough Shoal will be determined as rocks under UNCLOS
Article 121 and be entitled to a 12 nm territorial sea, but no EEZ or continental shelf. Collectively, such determinations will have the effect of severely restricting Chinese attempt to claim maritime features beyond what are provided by UNCLOS. This outcome should not be a surprise should it eventuate because the nature of maritime entitlements are clearly outlined by UNCLOS.
The major curve ball in my view would be Itu Abba. Whilst not part of the dispute, information was sought by the PCA from ROC. There are two possible outcomes that I could speculate. The first is that the PCA would not make a determination on whether Itu Abba is an island or not. I think such an outcome would not be desirable and its residual effect is not helpful towards future resolution of the disputes in SCS. The second possible outcome is that the PCA would determine that Itu Abba has the status of an island but its maritime entitlement is restricted to a 12 nm territorial sea. Such a determination will be highly controversial but is not without legal precedence as established in the cases of Nicaragua v. Colombia (2012) and Romania v. Ukraine
(2009). The main reasoning being adjacent land coastlines dominate the tiny coastline of an island.
(d)The legality of China restricting the Philippines economic access to its EEZ i.e. Scarborough Shoal. IMO, whilst there is a dispute over sovereignty, restricting one party access would be deemed inconsistent with UNCLOS especially when the use or the threat to use force is evident.