Is it permissible to lie to one's grandfather to prevent him from going to the mosque due to his poor hearing and eyesight, and is it permissible to tell him that another prayer is being held elsewhere, not at the nearby mosque, so that he prays at home?

It is not permissible to lie to the grandfather or hinder him from attending congregational prayers. Rather, it is obligatory to help him do so. A slight turning of the worshipper for a minor need does not invalidate the prayer nor is it disliked. Similarly, a congregant raising his head before the Imam, thinking the Imam has raised his, does not invalidate the prayer, but he must return to prostration and follow the Imam. The grandfather can be helped to hear during prayer by providing him with a hearing aid, or by gently nudging him when the Imam raises his head.