I don't think Walker knew Buchan. He did liaise a lot with Child over Buchan's ms which was then owned by I think Buchan's nephew, David Scott, Buchan having died in the 50s. Walker would have been a young man when Buchan died. While Walker was corresponding with Child he was completely in agreement with Child over Buchan's heavy doctoring of his ballads. When Child died and Walker met people like Greig collecting in the area, Walker began to backpedal on his stance over Buchan, wrote a bio of Buchan and started defending him.
Both Buchan's publishede works and Christie's are highly suspect as far as provenance is concerned. Christie used a lot of Buchan's suspect texts to bolster TBA and added his own second strains to many of the tunes.
This is all off the top of my head but I could easily check the facts for you as I have most of the published works on and by Buchan plus copies of his so-called mss. I say 'so-called' because when we think of mss we normally think of field notes with info on the singers etc., but in Buchan's case these don't exist. All of the items referred to as mss of Buchan are actually publishers' proofs.