it is not autoignitition that is the problem (plastics would have gone up in smoke too if it reached that sort of temperature).
But rather the paper tends to hold moisture more than the felt pads or rubber pads, and in environment with high moisture, this can lead to the magnesium alloy to discolour. This is a personal observation of mine.
Well... It's a bit hard to define "alleviate" in this case. The extent to which the palmrest flex is an engineering foresight is unknown to us; we can only speculate. Nevertheless, the creak I think we can all agree is a definite issue.
I've heard things about new policies regarding plastics, but I'm not entirely clear on that (I'm out of Lenovo warranty anyway).
I just added a rolled up chunk of paper under my keyboard that I missed before, and it's helped a lot. Maybe get some of that sticky tack and use that? It'd conforms to the shape of the roll cage and provide some degree of cohesion between the magnesium and plastic. (I'm not guaranteeing anything.)