" Suffice to say, reconstructing 1800-1900 year old religious sects to figure out what they each believed while history did its best to compound them into a single religion is really hard."
It is ! and of course the old JW view, and that of many Bible believers, that there was a uniform and united belief system accepted by the Apostles and others in the 1st Century is nonsense. There were various groups who followed Jesus, and disagreed on Doctrine.
The Apostle Paul was of course very different in his beliefs to those of the the Christians in Jerusalem, and he is at pains to point this out, in his genuine writings. This is obscured by the fictional Acts of the Apostles and by later writings pretending to be by Paul.
What he meant by " Firstborn from the dead" is probably as stated in previous Posts on this Thread, but would the other Christian groups have agreed with him ?