Is it permissible to say "And by the secret of the rank of the Fatiha," then recite the Fatiha, then send blessings upon the Prophet in the form of "O Allah, bless and grant peace to our master Muhammad, who opened what was closed and sealed what preceded, the helper of truth with truth, and the guide to Your straight path, and upon his family as is due his immense قدر and magnificent مقداره" after supplication?

It is permissible to make tawassul (seeking nearness to Allah) through the secret of Al-Fatihah, because the secret of Al-Fatihah is the comprehensive meaning that Allah intended and for which He revealed the Books. It is the speech of Allah and one of His attributes, and tawassul through the attributes of Allah is legislated. However, it is better to avoid this specific wording (secret of Al-Fatihah) as it was not reported from the Salaf. As for concluding dhikr (remembrance of Allah) and du'a (supplication) with the recitation of Al-Fatihah, it has no basis and is an innovation (bid'ah). Conversely, concluding them with prayers and salutations upon the Prophet, peace be upon him, is legislated. One must avoid the "Salat al-Fatih," which is an innovation from the Tijaniyya order.