Was Iblis’s whispering to humankind a compulsion for them to descend to Earth, because Allah willed it from the beginning and gave them an enemy to test their will? Is the existence of a demonic companion a compulsion for humans, and therefore is Iblis and the devils’ entry into Hellfire an injustice?

Iblis was not compelled to whisper evil suggestions, nor was Adam compelled to respond to them. Both occurred by Allah's preordainment and decree. Every act of disobedience committed by a servant is by their choice and acquisition. They are both guided and given choice simultaneously, and facilitated for what they were created. This is the secret of divine decree that the human intellect cannot fully grasp.