Is it obligatory to repeat the ‘Isha’ prayer if the worshipers differed concerning the Imam’s recitation of Surat Al-Fatiha, whether by omitting a verse or stammering in it, bearing in mind that the worshiper was not focused during the prayer?

If doubt occurs after completing the prayer, there is no need for a make-up prayer (qada'), and your prayer is valid, because doubt after finishing an act of worship does not affect it. Imam Shafi'i differentiated between doubt during an act and doubt after an act, and he did not mandate repetition for the latter, in order to avoid hardship. Furthermore, jurists have stipulated that if the statements of the worshipers who alert the Imam differ, their statements are disregarded and not given attention.