I repeat: Ferneyhough's "musical thought" is inseparable from the notation he invented. If you translate his notation by simplifying it, you distort the "musical thought" (I like this expression!).
Of course, nobody crowds in to perform Ferneyhough. In Italy, he is never listened to (no more than five or six Italian quartets come to mind that are capable of tackling the piece being discussed). I don't think he has many fans in England either. He has more luck in France, Switzerland, Germany, and the Netherlands.
Regarding the squared ffff on a rest, could you indicate which measure you are referring to? At m. 90 it refers to the figure following the rest, likewise at m. 103 and 124.
Regarding the idea of "playing with an intention" which is concretely unachievable, I am reminded of Charles Rosen: "And where Beethoven, in Opus 90, asks for a successive crescendo and diminuendo on a single sustained note, the instrument that can realize this has not yet been invented".