Waltz & Engle's Traditional Ballad Index extensively discusses Garfield, the assassination, and Guiteau's state of mind:

https://balladindex.org/Ballads/LE11.html

I don't see the two additional stanzas fitting into Lunsford's "Mister Garfield." They appeared in texts of the Guiteau ballad.

Is there still another "Garfield" song?