Why not dismiss the trinitarian belief outright

[GUBU]

What evidence is there that the NT community disagreed on the oneness of God?

Well, for example, Marcion of Sinope, the inventor of the first christian canon and one of the earliest christians (he lived in the first half of the second century, right at the birth of christianity), believed that there were two gods, an evil one called yhwh who was depicted in the jewish scriptures and a good, hidden, god who sent Yeshua as his messenger. He also denied that Yeshua had an actual physical body, that what the disciples saw was simply an image.

That's a big disagreement on christian fundamentals from what is now considered canonical.