ما الدليل من القرآن والسنة على حرمة اتباع جنازة غير المسلم إلا للضرورة كالقرابة؟

There is no definitive proof regarding following the funeral procession of a disbeliever, but there is definitive proof prohibiting praying for them and supplicating for them. Some jurists have gone so far as to prohibit following it because it resembles praying over the deceased, while others have permitted it based on narrations from the Companions and the Tabi'in, as a number of them attended the funeral of Umm al-Harith ibn Abi Rabi'ah, who was a Christian. Some scholars have permitted following the funeral procession of a disbeliever as a courtesy, likening it to a relative or neighbor, by analogy with the permissibility of visiting and consoling them.