Subregular Fourier-coefficient conjecture for relevant unitary Arthur packets

Let $G_n={\mathrm {U}}_{n+1,n-1}$ be defined over $F$ with $F$-rank $n-1$, and let $G_n^*={\mathrm {U}}_{n,n}$. Let $\phi$ be a generic global Arthur parameter of $G_n^*$ that is $G_n$-relevant, meaning that the global Arthur packet $\widetilde{\Pi}_\phi(G_n)$ is nonempty. Let $\underline{p}=[(2n-1)1]$ be the partition corresponding to the $F$-stable subregular unipotent orbit. Subregular Fourier-coefficient conjecture. There exists an automorphic member $\pi_0$ in $\widetilde{\Pi}_\phi(G_n)$ with a nonzero Fourier coefficient associated to $\underline{p}$. This conjecture supplies the refined packet property needed to extend the paper's non-vanishing results from low-rank quasisplit groups to the general situation.