Malcolm is a name DH and I considered for our DS and we kind of had the same issue with it, as our surname ends with an "-un" sound. Rhymey, but not as bad as Byrum. :o)
However, I think it sounded better as a fn (in our case, at least) than a mn for some reason. If you do decide to use it as a fn, may I suggest my Grandpa
Malcolm's nn, Mac? Really, it's the nn Mac that completely sold me on the name
Malcolm.