It appears the question of Margaret Beauchamp had been addressed a couple of years ago, but I see she's still connected as his wife in spite of the curator note that still says:
Sir Walter Buchanan of that ilk, 11th Chief of Buchanan
x Margaret (NOT Beauchamp)
I had read at this link that this Walter's wife was named Margaret but it doesn't give her maiden name:
"11th – Sir Walter Buchanan,[69][70] (Guthrie Smith identifies him as 5th Chief and Buchanan of Auchmar identifies him as 11th Chief) married Margaret and had three sons..."
https://en.wikipedia.org/wiki/Clan_Buchanan
So it seems she may have actually been named Margaret, just not Margaret Beauchamp.