What is the reason for the impermissibility of inserting a finger into the vagina of a menstruating woman, even though Al-Nawawi's statement in Sharh Muslim indicates the permissibility of enjoying a menstruating woman even if there is some blood on the spot?

There is no contradiction between what An-Nawawi mentioned and the fatwa (definition). An-Nawawi divided enjoyment with a menstruating woman into three categories: 1. Intercourse: Unanimously forbidden. 2. Direct contact above the navel and below the knee: Unanimously permissible, and there is no difference whether there is blood on the spot or not. 3. Direct contact between the navel and the knee, excluding the private parts (vagina and anus): There are three opinions: the soundest is prohibition, the second is disliking (tanzihan) which is the strongest in evidence and the preferred view, and the third is permissibility for one who can control himself from the private parts.

In summary, enjoyment with a menstruating woman in the private parts is forbidden, whether through intercourse or otherwise. This is confirmed by the hadith (definition) of Ikrimah: "That the Prophet, peace and blessings be upon him, when he wanted something from a menstruating woman, would place something over her private parts," and the hadith of Masruq from Aisha, may Allah be pleased with her: "What is permissible for a man from his wife when she is menstruating?" She replied: "Everything except the private parts."