Is it obligatory to pay blood money (diyya) if there is a possibility of error in diagnosing the death of a patient who did not undergo complete eye tests?

Ward off whispers [of doubt]. The signs you mentioned are sufficient to determine a person's death. Among the signs that confirm death are: cessation of breathing, fixed gaze, parting of the lips, and drooping of the feet. If you have confirmed his death by these signs, and your doubt is merely an illusion and a whisper, then no expiatory payment (diyah) or atonement (kaffarah) is obligatory upon you, unless the contrary becomes evident.