As you can see T. Kerwin did not find the Christian Planck family in 1870 and did not find Fred Planck with the Plancks in 1880 in Porter, Cass Michigan. You mention finding Fred with Christian and Emma Planck "in 1870 or 1880"- would you mind posting a transcription of the record you have?
There is a Freddie Roof born about 1867 in Michigan found in the 1870 census in Porter, Cass, Michigan. (Maybe this is the census record you had meant?)
Catherine (Eberhard) Roof (wife of Daniel) is still living in Porter, Cass, Michigan in 1910, age 91, living with Rachel and Samuel Skinner (Rachel being her daughter). I haven't checked back in time to see where the Roof family might connect with your Planck/Plank or Duesler/Dueslar family.