What are the implications for the Imam, the congregants, and the deceased if the Imam erred in the funeral prayer by saying the Tasleem after each Takbir, and the congregants were unaware of the rulings of this prayer?

The aforementioned funeral prayer is invalid due to its deviation from the prescribed method. It must be repeated for the deceased, even if they have been buried, unless more than a month has passed since then, according to many scholars.