I have written a new version of nicematrix (v. 6.0 of 2021-08-10) which solves that problem (by using the technique used by touhami in its first code). It's now possible to use \multicolumn in a {tabular} nested in a {NiceTabular}.
Professeurs de mathématiques dans le secondaire, connaissez-vous le package ProfCollege de Christophe Poulain ? Il est aussi utile que bien documenté !