I have no special knowledge of the specific collaboration at Old Elm. I’m just weighing in here to point out that such collaborations have happened many times in golf design.
As with MacKenzie in Australia, many clubs would have tried to interest Colt in helping out at their club, once they didn’t have to pay to get him to America in the first place. Some would have been redesign projects and some new ones; Old Elm was just in between as it hadn’t been built yet.
Colt (like MacKenzie) would have welcomed having someone to supervise construction. Ross might have just wanted to stay involved and collect the rest of his fee, or might have thought about doing more collaborations. But then a World War intervened, and then Alison came to America to represent Colt, and Ross became the competition.
The reason to bring Ross to The Country Club of Detroit was surely to indoctrinate him in how Colt liked to build things, not to get Ross’s help there. I have done this often with new construction people.
As I said, it’s not unusual. You could make the case that Michael Clayton and I had a similar collaboration at St Andrew’s Beach, or Jack Nicklaus at Sebonack. Martin Hawtree used Tony Cash more’ screw to build his work in Australia. Robert Hunter and Perry Maxwell both had contracts for projects in America before MacKenzie’s arrival, that became collaborations when they decided to partner up (although, in all of those I know of, they stuck with Hunter or Maxwell’s routing).