To what extent is the hadith "Their heads are like the humps of camels; they will not enter Paradise nor even smell its fragrance" authentic? And what is the impediment to placing a large hair clip (qarrasa) in the hair under the hijab? And what is the apparent contradiction between this hadith and the hadiths about entering Paradise with an atom's weight of faith?

The aforementioned Hadith is authentic. If the word "qurāṣah" refers to a hair tie or a hair bun, then it is permissible. The wisdom behind prohibiting this action might be that it is a form of tabarruj (displaying one's adornments) and an invitation to tempt men, and Islam blocks the means leading to evil. The statement that women will not enter Paradise nor smell its fragrance is a form of severe admonition and a clear indication of the seriousness of tabarruj, and it does not mean they will be entirely barred from entering Paradise. One must beware of belittling sins, and a Muslim should inquire about religious rulings with a desire to understand, not with an attitude of objection.