Sven, I've searched high and low for the Eugene Grace Estate Course but, to date, haven't found anything. The earliest reference that I've found to such a course is an article in the 9/21/1945 Daily Ardmoreite which notes that Maxwell "skyrocketed to fame building many of the country's best known courses including...Eugene Grace's private golf course at Philadelphia". I suspect subsequent mentions of such a course are derivative of this initial reference. However, I haven't seen anything to indicate that Grace ever had an estate in Philly, much less a golf course there. Rather, his realm was in Bethlehem. Grace's personal history is very well documented. He lived in the middle of town in a mansion that doesn't appear to have been large enough for more than a hole or two. Old aerials show no signs of a course on that property nor have I found any mention of golf being played there. My best guess is that the 1945 article is just a very poorly worded reference to Maxwell's redesign of Saucon Valley. Grace ran Saucon as a virtual autocracy, so perhaps it was confused as his estate. Maxwell worked there from roughly September of 1944 thru February 1945, which falls nicely into the timing of the article. Moreover, Saucon fits as a well known course whereas a private estate course doesn't. Hopefully I'm wrong and concrete evidence eventually turns up that Maxwell designed a private course for Grace. I'll keep looking and would love to see anything anyone else has. But I'm not optimistic.
Depending on how you define an estate course, you might also want to add Overhills in NC to your list. Chris Buie's wonderful thread on Overhills has complete historical information. While I don't believe it was originally conceived as an estate course, Overhills became a Rockefeller family retreat shortly after opening.