How can one reconcile absolute certainty in God, which leaves no room for doubt, with a servant's belief that God's wisdom may necessitate the occurrence of what one dislikes, as in examples of healing from illness or facilitating Hajj?

Certainty is not sought in the examples mentioned, but rather in the knowledge of Allah and belief in Him, in the Last Day, and in the truthfulness of the Prophet, peace be upon him. One must be certain of Allah's wisdom, even if it is not apparent, and that what Allah has decreed is wisdom and benefit. One must supplicate with certainty that Allah will ordain what is good, for Allah alone knows what is good and beneficial. The servant must have a good opinion of his Lord, take the necessary means, and be content with what Allah has chosen for him.