Sorry for posting this info after the fact, but I thought it might still be of interest.
The marathon course (42.195km) is USATF certified, but the individual leg distances are not (although they are accurate), since they are not record-eligible anyway (first leg because it is not a record distance, and the others because they have a running start). Below is a link to the certification map.
https://www.certifiedroadraces.com/certificate/?type=m&id=635
The course was certified in 2020, and the order of the legs was different for 2021. Below is a map link with the 2021 exchange locations and mile splits.
https://www.dukerdog.com/Michigan_Pro_Ekiden.png
Below is a link to the elevation profile. Some legs gained elevation and some lost, but none of them was flat.
https://www.dukerdog.com/elevation_profile.png
Finally, the park trail was not closed to the public during the race. If the race were held on the weekend there would be a large number of walkers, runners, and bikers on the path during the race. The park is not likely to close the path for 2+ hours on the weekend for an event that 95% of park goers don't care about.