Is it permissible for a Muslim who has converted from Ismailism to the Sunni path (Ahl al-Sunnah wal-Jama'ah) to pray in Ismaili mosques, performing the Sunni prayer without folding the hands or raising the voice with "Ameen", out of fear of his family, or should he pray at home?

There are two types of prayer behind an innovator: If his innovation is an act of disbelief (kufr), such as someone who seeks help from other than Allah or believes the Qur’an is incomplete, then prayer behind him is invalid. However, if his innovation is not an act of disbelief, such as exaggerating in dhikr (remembrance of Allah), then prayer behind him is valid, though disliked (makruh).

If it is not possible to avoid praying behind an innovator imam for fear of harm, then it is permissible to pray behind him with the intention of praying individually. In this case, the worshiper follows the imam in apparent actions (like standing and bowing) but intends to pray for himself without following the imam, and his prayer is valid thereby.