Calling that interpretation of the Godhead an error is a matter of opinion and how you interpret what you're reading in the Bible. Oneness and Trinity are either very far apart or everyone saying the same thing in different ways depending on what end of the spectrum who you're talking to is. I've studied both pretty extensively and have come to the conclusion that we humans probably make it far bigger of an issue than it is if it even is one.

...and it's a total exaggeration to say in the UPC there are as many songs about Oneness as there is about God. That is simply not true. I grew up in the UPC... and I can think of about 4 songs about the Oness off the top of my head. ...and only one of them is very overtly oneness... the others are about God but I guess you could call them Oneness since one, for example says after a bunch of words about worship on the very last line says, "the great creator became my savior, and all God's fullness, dwelleth in Him." Big deal. Many churches sing, "Holy, Holy, Holy" which ends in "blessed trinity."...but that's not what the song is about.

I do agree with you about the issues with legalism, the UPC being the only church going to heaven, and in many of the churches the list of rules all becoming heaven or hell issues which causes a lot of guilt... and the pride in seperation from the other churches.