Is it permissible for a worshiper who believes that the Imam is combining prayers without a valid Shar’i excuse to intend a supererogatory prayer (nafilah) instead of a fard (obligatory) prayer when the Imam combines them? And is the reward for praying on time, even if the congregation is small or one prays individually, greater than the reward for praying in congregation when the combining of prayers is unjustified?

Combining two prayers without a valid excuse is a grave sin. The second prayer, when combined with the first by estimation without a valid excuse, is not valid. This is what the two Sheikhs transmitted from the author of Al-Uddah and affirmed. It is forbidden for you to combine prayers with them if you know that they are combining without a valid excuse. You must forbid them from this evil. If they do not respond to you, then you should leave the mosque after their first prayer and do not combine with them. It is not permissible for you to pray the second prayer with them with the intention of it being a supererogatory prayer. When the time for the second prayer arrives, pray in the mosque with those who pray at the correct time, or in another mosque, or in your home in congregation. If you do not find a congregation, then pray alone at the correct time.