There are various versions of this ballad and is one of the best known Scottish Border Ballads. I have not come across the exact version from p.j. but in answer to her questions:-
Marrow can mean- match, mate(husband or wife), an equal in rank or stature or battle.
Yarrow is a valley, a river and a small community in the Scottish Border Hills (Southern Scotland) which is still predominantly an area of hill sheep farming.
Doubt used in this setting means that "I am sorry but I think it will" bring you sorrow. It is showing regret but resignation to an event.
I can not make real sense of the last verse and Da in the fifth verse is not in the Scottish vocabulary.
Somre of the versions use noblemen instead of brothers, some say he was thrown into the yarrow river after he was killed and she used her long hair to pull him out, some say he killed 3 brothers, and wounded three others before he was killed by one who sneeked up behind him. the song continues that she blamed her father for not wanting her to marry the ploughmanand died of a broken heart. This is not the case as it has been shown that she got over the episode and married someone else. Your version seems to be an amalgumation of other versions.