In thread 15922 Sandy Paton confirmed that Plenn wrote the final verse.

I have not seen that anyone has ever tried to explain the substitution of "Love" for "Jesus" or "Saviour". Pete Seeger could have changed it to make it more universal, or Plenn could have changed it to bring it more in keeping with the anti-McCarthy sentiments of the final verse.

When churches add it into their newer hymnals I wish they'd switch it back.