Richie,
I thought c1840 off the top of my head was pretty good for 1831-36 but there you go. That little c. means approximately. It's not meant to be accurate.

Yes, I have a copy of the Swindells text. It's from the Madden Collection as is the Batchelor copy. I'll scan it and email tomorrow.

I agree with you re Salcomb song. It is rather too poetic to have come from tradition and is very likely from Masefield's pen. I certainly can't find any earlier copies.

Yeats was also noted for rewriting ballads from tradition.