It would not surprise me that Mrs. Rutherford left her husband due to his roving eye.
Malcolm's trip may have been an attempt at reconciliation on both parts. Many sons who grow up without a father will grasp at an attempt to get to know him. Rutherford on the other hand may have been quite willing to throw an all expense paid trip at his son in an effort to win him over to his side of the difficulties between his parents. But the trip may have simply validated Mrs. Rutherford's opinion of her husband which would have left a permanent rift between the father and son.
Remember too that divorce was not widely accepted especially in religious circles so separation would have "saved face".
If I recall there is an image (I have it at home but I won't be home for a few days) of Malcolm where he is noted as either Russell's ot Rutherford's secretary (I think it was secretary - anyone got the image?). So Malcolm was involved with the Bible Students.
I suspect that like many others who left the Bible Students when Rutherford illegaly took over the presidency Mrs. Rutherford and her son may have gone with them.
Just guesses on my part but I think they make sense out of Rutherford's nonsense