You can often find old words in an unabridged dictionary, especially an older one. I have one from the 1930's which I bought for $1 at a library sale. (They should have kept it.)

"Marrow" means match or equal. It rhymes with arrow.

A plooman would indeed be a plowman. I was at a Scottish concert recently and heard this very pronunciation. Of course, this means that the lover was a peasant, which is why the nine men killed him.

I'm sure "looed" is "loved." "Looved" is a more common variation.

I think the reference to "her lover John" is a silly booboo. This kind of thing should be avoided. The songs are all clear that John was her brother. Keep in mind that from the days of King John (a horrible man) the name John has never been used for an English king. I think that the reference to her brother John, who stabs the poor guy in the back, is an echo of this.

"I doubt" can mean "I fear," rather than the modern "I don't believe." I don't think it ever means "I'm sure."

Yarrow is the place that the plowman is from. The dens are ravines, and they are sad and dull, or "dowie." Just the sort of unproductive land that a conquered people (now the peasants) are likely to be left with. I like to think that it is on the dark, north side of a rugged Scottish mountain.

When I sing this song, I like to stop after the father's speech about the lover's death. The other verses add hardly anything to the story. This is perfectly legal.