Is it obligatory to adhere to diacritics (tashkeel) when reading fatwas and religious books other than the Holy Quran, or is correct recitation only obligatory in the Holy Quran?

There is no evidence for the obligation of diacritization (tashkeel) when reading the words of scholars, nor for the prohibition of grammatical error (laḥn) therein. In fact, grammatical error in the Friday sermon does not invalidate it. Learning the Arabic language is a شرعي (Shar'i) requirement and a communal obligation (fard kifayah). It is incumbent upon one who is able to read fluently to strive to protect their tongue from grammatical error, as this preserves the method of understanding the Qur'an and Sunnah and emulating the Arabs. The predecessors used to discipline their children for grammatical errors, and people in the past avoided grammatical errors in both writing and reading. Avoiding grammatical errors is a sign of majesty and refined taste.