The ending plural nūn of مُسْلِمُونَ disappears in iḍāfa.
The «نِ» of جَاءَنِي is the protective nūn attached to the verb, a different element.
The shadda in مُسْلِمِيَّ reflects assimilation of two yāʾs, not assimilation of nūn.
Three distinct concepts: deleting plural nūn, protective nūn, and yāʾ assimilation.