Is the aforementioned will Islamically valid and enforceable, and who are its current beneficiaries, or is it void, and the inheritance must be distributed according to Islamic law?

The Sharia court is the competent authority to rule on such cases. For the benefit of understanding, acting upon a mere handwritten endowment (waqfiyya) is not valid unless there is no suspicion of forgery and the probative force of the endowment is trusted, especially after the death of the endower and the witnesses. The Mejelle (Ottoman Civil Code) stipulates that handwriting and a seal alone are not to be acted upon unless they are free from suspicion of forgery and fabrication, in which case they can be acted upon. Similarly, Sultanic decrees and entries in judicial ledgers and registers are to be acted upon if they are secure from forgery. As for the endowment deed (waqfiyya), it is not to be acted upon unless it is registered in a trustworthy court record. Ali Haydar explained that handwriting or a seal alone is not sufficient for a ruling due to the possibility of similarity or forgery, and they are not to be acted upon together unless there is no suspicion of forgery and their content is proven by a valid legal argument or a just testimony.