A Chumash has to go on top. Many Poskim hold that a Gemara has to go on top of Nach, but others disagree. All agree that a Nach with Meforshim goes on top of a Gemara because then it contains Torah Shebiksav as well.
Englishman: I don’t think it’s so true that you can’t use a lower Sefer as a prop. I believe R’ Matisyahu Salomon has a piece on this in his Matnas Chayim on Moadim, but I don’t remember his sources.
On the ball: I don’t think that’s a S’vara that the Poskim bring down.
Curiosity: That makes sense but I don’t recall seeing any Poskim say it.