What is the ruling on an individual praying alone behind the row in congregational prayer if the worshippers intentionally distance themselves from each other in the rows and leave gaps, such that the first row is not completed, then the Imam gathers them together again in a complete row for the Witr and Shafa' prayers? And is this considered an innovation (bid'ah)? And is the prayer invalidated due to the presence of hadiths that mandate the completion of rows?

It is not prescribed to intentionally avoid straightening the rows, or leaving gaps between them, or starting a new row before the previous one is complete in any congregational prayer. This contradicts the guidance of the Prophet (peace be upon him). Straightening the rows, completing the first row then the next, and closing gaps are a recommended Sunnah according to the majority of scholars. If worshipers are negligent in this intentionally, their prayer is valid, but they miss out on a great good. Their action is not considered an innovation if it is due to laziness and not out of belief in its legitimacy. Some scholars are of the opinion that straightening the rows is obligatory. There is no difference in the ruling on straightening the rows between Tarawih prayer and other prayers, as the general evidence includes every congregation. As for the prayer of an individual behind the row without an excuse, it is valid but disliked according to the majority of scholars.