Is it considered a sin for a حافظ (one who has memorized the Quran) to doubt the vocalization of words (such as رفع and نصب) during recitation, given that he does not lead prayers or recite for anyone, and it is not currently possible for him to review with a شيخ (religious scholar)?

It is not permissible to deliberately recite while doubting what you are reciting. Rather, certainty or a strong presumption of the correctness of what is being recited is required. If you doubt something in the Qur’an, it is not permissible for you to recite until you are strongly confident of its correctness. We advise you to listen extensively to the Qur’an from proficient shaykhs and reciters via audio recordings, as this will help you regulate the pronunciation of the Qur’an, facilitate memorization, and avoid errors.