Is it permissible to ask Allah by the blessing of the righteous, such as saying: "Forgive me, O Lord, by the blessing of so-and-so"? And is it permissible to pray behind an imam who writes amulets and incantations for people for healing?

Seeking intercession through the blessing of someone, or their status, or their right, or their person, is not shirk (polytheism), but it is an innovation (bid'ah) and a means to shirk, because intercession is an act of worship based on divine command and prohibition. The permissible forms of intercession are through the names and attributes of Allah, through monotheism (Tawhid), and through righteous deeds. Prayer behind someone who practices this form of intercession is valid, but they should be advised and taught. If they do not respond, they should be replaced with someone else. Hanging amulets and talismans is a form of shirk, and the Prophet, peace be upon him, forbade it, even if they are from the Quran, according to the preponderant opinion. As for incense used in treating some diseases, it is permissible as long as it is not accompanied by serving jinn or claiming knowledge of the unseen, for this is a great evil that must be prevented.