Tom
That is my understanding too - I've never seen anything about Mackenzie at Sunningdale either. Perhaps that is what Crane did in adopting the convention, perhaps he knew something specific too.
With respect to the Eden, he did generally avoid saying anything about it other than that Colt was the architect. However, I did find this entry in 'Spirit of St Andrews' p 10, where he states clearly,
"At St. Andrews there are two other excellent courses - the New and the Eden (designed by Mackenzie and H.S. Colt)".
So there it is in black and white, and he even lists himself first! I don't believe this comment of attribution has really been noted by anyone before to my knowledge. I know that you are convinced that some of the Eden greens are obviously non-Colt, so I suppose this attribution would come as no real shock to you. Some independent evidence of his involvement on the Eden would be great too, but this may be all we have to go on.
As for Sunningdale New, perhaps the same applied and he did collaborate with Colt there too.