Does not the eternal torment of non-believers in Hell contradict God's being more merciful to His servants than a mother is to her child, and His being just, considering the brevity of their lives in this world?

The owner's disposal of his property is just, and Allah is the most just of owners. He is not questioned about what He does, and He is the Most Merciful of those who show mercy. Allah created His servants for His worship and instilled monotheism in their nature. He sent messengers to clarify the path for them and to establish proof against them. Whoever deviates from this and persists in polytheism and disbelief deserves eternal damnation in the Fire. Even if they were returned, they would revert to what they were forbidden from. One must think good of Allah and know that Allah wrongs no one. The way to know the unseen is through the Qur'an and Sunnah (definition), not through intellect.