That last line, Lighter, is not a plea for future help, it's in the historic present tense, God's helping us because he's the father of the child I called upon to help. It's the moral of the song, and absolutely essential in the form. If it were as you have it, it would be in a conditional tense, but it's not.

The point is that it closes one of the uncertainties down, that if it wasn't Josephs' then perhaps it was Nicodemus', or Josua's, or...

What might be interesting is to chase the tree lore thread down. Was this imported from a celtic lore line? This is where the ur-ballad thought comes from, that just as Christianity hijacked the pagan temple sites, and Saturnalia for Christmas, and the Eucharist from the Jewish sabbath service, and ... (the list is long), was this symbolism something in a different song?