A retract theorem for nilpotent Lie groups

Let $G= \exp(\g)$ be a connected, simply connected nilpotent Lie group. We show that for every $G$-invariant smooth sub-manifold $M$ of $g^*$, there exists an open relatively compact subset $\mathcal{M}$ of $M$ such that for any smooth adapted field of operators $(F(l))_{l\in M}$ supported in $G\cdot \mathcal{M}$ there exists a Schwartz function $f$ on $G$ such that $\pi_l(f)= \op_{F(l)}$ for all $l\in M$. This retract theorem can then be used to show that for every Lie group $\G$ of automorphisms of $G$ containing the inner automorphisms of $G$ with locally closed $\G$-orbits in $\g^*$, the proper $\G$-prime two-sided closed ideals of $L^1(G)$ are the kernels of $\G$-orbits in $\hat{G}$.