What is the ruling regarding the prayer of a congregant who prayed behind me, knowing that I intentionally led the prayer in ritually impure clothing and then repeated the prayer individually?

You must repent to Allah Almighty and not intentionally return to praying in a ritually impure garment. You must also repeat the missed prayers. As for the prayer of the congregant next to you, it is valid, and he does not need to repeat it, even if he sat on the impure spot and was unaware of it. Regarding the prayer of a congregant who finds two people praying in congregation, there is a detailed explanation for this in the aforementioned fatwas.