Is it obligatory to merge the nūn of (nasta‘īn) when connecting it to (ihdinā al-ṣirāṭ al-mustaqīm), and would the prayer be invalidated if the nūn of (nasta‘īn) is pronounced with a fatḥa and connected to (ihdinā)?

When connecting "nasta'in" (we seek help) with "ihdina" (guide us) in Surah Al-Fatihah, the "nun" of "nasta'in" must be pronounced with a dammah. Opening the "nun" (i.e., pronouncing it with a fathah) does not invalidate the prayer because it is a grammatical error (laḥn) that does not change the meaning. However, intentionally opening the "nun" is forbidden because it constitutes a distortion of Allah's Book. If one does so intentionally, their prayer and recitation remain valid, and this is the view of the majority of scholars. Moreover, it is not permissible to commit grammatical errors in Al-Fatihah, even if the meaning is not altered.