I'm glad someone honestly asked the question. It's interesting and should be talked about because the issue is confusing and worrisome to many.
That said, I'm not sure the discussion has been entirely respectful. Just sayin...
I don't think this is true. The Didache suggests otherwise and it was written between the years 65-80.
"And concerning baptism, baptize this way: Having first said all these things, baptize into the name of the Father, and of the Son, and of the Holy Spirit, in living water. But if you have not living water, baptize into other water; and if you can not in cold, in warm. But if you have not either, pour out water thrice upon the head into the name of Father and Son and Holy Spirit."
Today I've consulted a few articles/podcasts on this subject by respected canon lawyers who have interesting perspectives on this subject. The idea here seems to be less about the pronoun and more about the intention of the priest. Consider that in this instance it is incredibly easy and simple to just get this right and avoid trouble altogether. The person changing the language is doing so intentionally. It's not a passive form or some cultural manifestation or linguistic ambiguity. If you're replacing I with We in the western church within the last 50 years, it's on purpose. So it's as if the Church is saying that if you're the sort of person who is intentionally doing this incorrectly, your intentions cannot be trusted and therefore neither can the validity of the baptisms.
Correct. Priests do not have their own priesthood. They are ordained into the priesthood of Christ. I'd shy from the term intermediary. Christ is the one Mediator. From the Catechism...
So within the context of the sacraments, it is not the priesthood of the man before us that we encounter, but the very priesthood of Christ himself. Could God have done this in others ways? Sure. But this is the way that has been given to the Church and as physical beings in time and space, it seems fitting that the Incarnate Lord comes to us in physical means that allow us to encounter his grace efficaciously.