Why doesn't Allah admit us all to Paradise and unite our hearts, seeing that He is All-Powerful over everything? And what does He gain from tormenting His creation? Is He not the Most Merciful?

Allah Almighty is All-Powerful, All-Merciful, All-Knowing, and Just. He possesses perfect wisdom in His creation. He created creation with disparities in everything to test them and to distinguish the wicked from the good, the truthful from the liar, and the believer from the hypocrite. This can only occur through trial, tribulation, and purification. He surrounded Paradise with hardships and difficulties, and Hellfire with desires and pleasures, so that reaching Paradise would necessitate overcoming hardships and avoiding desires. Had He willed, He would have made humanity one believing nation, but His wisdom necessitated creating for people free will and choice, for which they will be held accountable, and to manifest His justice and wisdom, and what lies hidden in human nature of good and evil.