Is the ḥadīth "The اختلاف (disagreement) of the scholars of my community is a mercy" authentic, and is Iblīs considered a believer despite his testimony to God's Oneness, and how do we reconcile this with God forgiving whoever has an atom's weight of faith in their heart?

Iblis is not among the believers; rather, he is among the disbelievers, as Allah Almighty says: "And [mention] when We said to the angels, "Prostrate to Adam," so they prostrated, except for Iblis. He refused and was arrogant and became of the disbelievers." And he and his followers have been condemned to Hellfire, as Allah Almighty says: " [Allah] said, "Get out of it, reviled and banished. Whoever follows you among them - I will surely fill Hell with all of you."