What is the ruling on writing a verse or a word backward, such as "Fattah" in this manner? And is that considered one of the major sins?

It is not permissible to disjoin the letters of Quranic verses or words, nor to intentionally reorder their words by advancing or delaying them. This is because it involves distortion and alteration of the meaning, and a contravention of the Quranic orthography, the adherence to which is unanimously agreed upon as obligatory. Scholars have denounced the method of some charlatans who create a ruqyah (exorcism) table that includes disjoined letters, and they have called for resorting to the শরী‘আ-compliant ruqyah instead.