Is it obligatory for the vulnerable Muslims who have only one shaykh to request a detailed and elaborate proof for the fatwa, due to their discomfort with brevity?

The Mufti is not obligated to mention the evidence with the answer, especially if the evidence is not definitive or if the questioner is ignorant of the aspect of its implication. The layperson should not demand evidence from the Mufti. It is worth noting that elaborating on the evidence in every fatwa received from all corners of the world would be lengthy, and the principle in fatwas is brevity and conciseness. As for the evidence for the ruling issued in Fatwa no. 73659, it is the principle of original innocence (al-bara'ah al-asliyyah), meaning that the original state is the acquittal of a person's responsibility from burdens and sins, and the original state of actions is permissibility and lawfulness unless there is an impediment.