Does the funeral prayer become void if the worshipper supplicates for the deceased in the second takbīrah instead of reciting the tashahhud, then realizes the mistake and repeats the second takbīrah and recites the tashahhud, resulting in five takbīrāt instead of four?

Firstly, the prayer upon the Prophet (peace be upon him) in the funeral prayer comes after the second takbeer, with the well-known formula used in the final tashahhud. This is a pillar of the funeral prayer for one who is not a latecomer. As for the latecomer, it is recommended for him to make up for the takbeers and prayers he missed, but his prayer is valid even if he does not make them up.

Secondly, if the worshipper makes a mistake and supplicates after the second takbeer instead of praying upon the Prophet, he should then offer the prayer (upon the Prophet) and then follow the imam and pronounce the third takbeer. This delay does not harm him. If he did this out of ignorance, there is no blame upon him.