FWIW, I think the OP should pursue this if they wish too. Quite often it provides satisfaction even if the outcome is does meet ones desired outcome. I did this with the Maui Ren just before they closed, and did get the satisfaction of knowing that JW Marriott's office took note. Perhaps the OP has already reached that point, but only the OP can make that assessment IMO.

As far as the JW Phuket itself, I stayed there back in December and it was an excellent stay. And yes, the breakfast buffet was included for this Plat and was by far the best I have seen at any Marriott. FWIW, I did not receive any letter or email offering 25% off at Hotel restaurants, nor did I book any "last time slot" spa services. I did book a massage package and was very happy with the massages and the spa in general.

I have no doubt that the OP encountered the problems stated, but I do believe it was probably an isolated case since we have no other reports of bad stays at this property. I am disheartened that Marriott is handling it in a manner contrary to their philosophy and summarily dismisses customer complaints to the responsibility of the hotel that bears their name.

I know Thailand is trying to build tourism business back after the recession and their annual political unrest, which last November made world-wide press due to the closing of the BKK airport. I think they would be eager to please customers and amicably settle any complaints.