Is it conceivable that a Jew or a Christian who has not received the message of Prophet Muhammad (peace be upon him) could be in a better state with God than a negligent and sinful Muslim, based on their sincerity in worship and acting upon the knowledge they possess?

The disbeliever who has not received the message (of Islam) and has not heard of the Prophet Muhammad, peace and blessings be upon him, his ruling in the hereafter is the same as that of the people of fatrah (a period when no prophet was sent). He will be tested. Whoever Allah knows would have obeyed if the command to enter Hellfire had come to him, will enter Paradise, and the Fire will become coolness and safety for him. As for worldly rulings, he is undoubtedly a disbeliever, and it is not permissible to prefer him over a Muslim, even if the Muslim is disobedient, because a Muslim is unequivocally better than a disbeliever.