Is what the scholars said—that the opening supplication (du'a al-istiftah) is Sunnah—correct, even though the hadith indicated its obligation? If so, what diverts it from being obligatory? And is the praise and glorification mentioned in the hadith not the opening supplication? And is this praise obligatory?

The opening supplication (Du'a al-Istiftah) is a Sunnah (definition), not an obligation, according to the general consensus of scholars. This is because the Prophet, peace and blessings be upon him, did not command it but merely recited it, as is clear from Abu Hurayrah's question to him about what he recited between the Takbir and the recitation. Furthermore, the narration concerning the command to the one who prayed badly to recite the opening supplication is not authentically established in the Two Sahihs. Even if it were established, it would be interpreted as a recommendation. Therefore, the opinion that the opening supplication is obligatory is weak and not relied upon in the followed madhhabs.