What is the ruling of the Shari'ah on withdrawing from congregational prayer when the Imam stumbles in recitation, and is the Imam sinful for this? And was it more appropriate for him to change the verses?

You should not have exited the prayer because of stumbling in your recitation after Al-Fatihah. Intentionally breaking prayer without a valid Shariah-compliant reason is forbidden (haram). Reciting after Al-Fatihah is a confirmed Sunnah, and the prayer is valid for whoever limits themselves to reciting only Al-Fatihah. Since you were ignorant of the impermissibility of breaking an obligatory prayer without an excuse, there is no blame upon you. You could have left the part that was difficult for you and moved to another Surah, or prostrated directly.