So good to see your postings as Marshall E has been a bit of a mystery to me. Or at least his paternal parentage. Yes the 1880 Pike County census(pg. 21) is where I found Marshall E Cottons name. I don't have a death date for his father Jesse James Cotton but I read an old posting from 1999 where someone said he died in the Civil War at Vicksburg. I don't know if there are any records backing this up but if that is true then the birth date of 1869 for Marshall E would make it impossible for Jesse James to be his biological father. The birth dates between Marshalls closest in age sibling would be Jesse D who was born about 1863 which would be a 6 year span between children which is odd. Of course there could always have been children who had died or a long seperation between the couple. Could Marshall have been illegitimate and she gave him the surname Cotton? I really don't know without a confirmation of death or any other kind of information on Jesse James. As for the 1880 census listing Marshalls father as being born in S.C. as opposed to N.C. I wouldn't put much stock in that as the same census lists his mother as being born in Alabama and she was born in N.C. as well. If you look at all of his census' from 1880 through 1920 his parents birthplaces are inconistent. Not uncommon when you never know who was giving the information to the census taker and if they just guessed etc.
As for the Grissette connection with Rebecca and Mary I have no idea. Like you said they are probablly cousins or something. I don't have much on the Grissettes as I am mostly researching the Cotton line.
Would love to know more about your Cotton line and please feel free to email me at email@example.com