Is doubt as to whether the beggar deserved the expiatory meal—after it was given to him—considered to affect the validity of the expiation, or does the principle that one should not assume entitlement require the expiation to be repeated?

The recipients of expiation (kaffarah) are exclusively the poor and the needy, and it is impermissible to give it to anyone else. One must be diligent when giving it to those who are deserving. If you doubt the eligibility of the person to whom you gave the expiation, you are not required to give it again, because the default assumption is the validity of the act of worship, and doubt after performing it does not affect it. This is especially true since Islamic law is based on what is generally prevalent, and what is generally prevalent among beggars is need.