Your brain is not rigid though - it can collect fluids and swell a tiny bit. Which essentially increases pressure inside it and if happens too much can be fatal. But that means you can squish a little bit more into without replacing mass - at least for a little while. Bones also regrow constantly, and with genital pressure and a lot of time you can reshape them.
I always assume that the microplastic is replacing body mass.
I dont think this is a valid assumption to make. I would see it more as your body working around the microplastics to do what it needs to do as best it can it does not have some limit as to the amount of mass it can use at any one point.