Is re-reading "اهدنا الصراط المستقيم" (Guide us to the straight path) correct when in doubt about dropping the letter Meem due to a sneeze, and is it sufficient to correct only the word "المستقيم" (the straight)? In both cases, should the prostration of forgetfulness (sujud al-sahw) be performed after the salaam or before it?

If you doubt having uttered a letter from Al-Fatihah, and you are not prone to whispers (waswas), then you must utter the word you doubted, or the entire verse.

Al-Fatihah is a pillar of prayer, and prayer is not valid without it or by omitting a single letter from it.

The general rule is that doubt concerning the performance of a pillar is like not performing it. However, if the doubt occurs after finishing the recitation of Al-Fatihah, it does not matter, because the presumption is that it was recited completely.

If the doubt is due to waswas (whispers from Satan), then turn away from it and ignore it. In this case, no prostration of forgetfulness (sujud al-sahw) is required.