Lets not get ahead of ourselves here. There are other claimants with navies much more capable and with much bigger budgets than the PN, yet they will still be unable to ''hold the PLAN'' off. Even if the PN gets 4-5 cutters and arms them with missiles and increases its budget tenfold - it will still be insufficient.
The whole point of modernising the PN is so it can better monitor its waters and the presence of newer and more capable PN assets will hopefully deter further Chinese ''activities'', it is not intended for the PN to engage in a shooting war with the PLAN as that would be silly. The best we can hope for is for the status quo to remain and put emphasis on diplomacy..........The Chinese are fully aware of the concerns that they have created and have no wish to escalate things further, as it is not in their interest to do so.
On the subject of the U.S., though there is a defence treaty with Manila and the Philippines is a non-NATO ally, whether or not the U.S will intervene remains to be seen. Should a clash in the Spratleys threaten the sea lanes in the South China Sea and effects the global economy, no doubt the U.S. will get involved but if a minor clash occurs, it remains to be seen if the U.S. will get involved as it has many things on its plate at the moment.