There is "one baptism" today (Ephesians 4:5). That one baptism is the baptism that saves, and that is water baptism.
Just because there was a departure from the Truth early on in Christianity doesn't make pouring a valid mode of baptism.
The historic Christian way of determining what is valid, includes a consideration of what the Church has believed from the earliest times, over long periods of time, in a widespread way, and in accord wth the testimony of the Fathers (the earliest teachers, who were themselves taught by the Apostles.)
This approach is required by the NT itself, which states that the Church has the power to "bind and loose" (Matt. 16:19 and Matt. 18:18) and has the authority to speak in Christ's name (Luke 10:16); that the Holy Spirit would lead the Church into all truth (John 16:13) and that "the Church is the ground and pillar of the truth"(1 Tim 3:15).
Thus, on the basis of the New Testament, we are required us to look to the Church for the basis of our belief and practice. The Church has the authority to determine the definition and the proper conditions for Baptism.
You wrote: "Just because there was a departure from the Truth early on in Christianity doesn't make pouring a valid mode of baptism."
My response is that (1) it's true that there are ancient errors as well as modern errors (antiquity doesn'r make a false thing true!), but (2) the practice of the Church, from the earliest times, approved by Councils and codified in the Church's own law, is certainly not in error. We know this because we know that the "gates of hell" will not prevail against the Church (Matt. 15:18.)
Never happened. The Holy Spirit doesn't go on 1300 year long lunch breaks.
The "departure from the truth" is much more likely to be recent than early.