The 15th at Muirfield was designed by Colt in the 1920's...long after the NGLA was built. Colt aslo redesigned the New course in the late teens/early twenties, and I believe his partner Morrison did some work there in the 30's.
Did Macdonald return to Scotland after building the NGLA? He was on the rules of golf committee...I reckon he had to come back to St. Andrews periodically, although I don't recall reading that he did.