How can we reconcile the Prophet's guidance to Umm Habiba, "You have asked Allah for appointed terms," when she prayed for Allah to let her enjoy the Prophet, her father, and her brother, with his own supplication: "O Allah, let us enjoy our hearing, our sight, and our strength"?

The two hadiths do not contradict each other. The first guides towards what is superior, which is asking for deliverance from the punishment of Hellfire and the grave, because it is an commanded act of worship, unlike supplicating for a long life. As for the second hadith, it includes asking for what aids in knowing Allah and obeying Him, which is the path to deliverance from punishment and the key to happiness. For the knowledge of Allah is attained through the senses, and strength is required for obeying Him. So, the path to salvation is for the servant to know his Lord and obey Him, and to ask Him for protection from punishment. Thus, both supplications share a single goal.