Minuscule representations and Panyushev conjectures

Recently, Panyushev raised five conjectures concerning the structure of certain root posets arising from $\mathbb{Z}$-gradings of simple Lie algebras. This paper aims to provide proofs for four of them. Our study also links these posets with Kostant-Macdonald identity, minuscule representations, Stembridge's "$t=-1$ phenomenon", and the cyclic sieving phenomenon due to Reiner, Stanton and White.