Is it obligatory to complete the row formed between the first and second rows for the funeral prayer, or is it permissible to pray without completing it if there is no space for the mourners in the front rows?

The Sunnah for the funeral prayer is that it be performed in congregation, though it is permissible to perform it individually. It is recommended that the rows be three or more, based on the hadith of Malik bin Hubairah: "There is no Muslim who dies and three rows of Muslims pray over him except that [Paradise] becomes obligatory for him." This is also supported by the action of the Prophet (peace be upon him) when he prayed the funeral prayer over An-Najashi. If the number of worshippers exceeds three rows, it is preferable to complete the rows, like in other prayers.