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I think can clear up a few items from your list. The Stells who migrated in 1835 to AR from Gwinnett Co, GA were Robert, his son John King, and Robert's brother Dennis Q(uimby) Stell. All three can be found on page 54 of the 1840 Conway Co., AR census. Also migrating to AR were William Stell (1850 census in St Francis Co., AR, page 48) and John Wesley Stell. William and John Wesley were both brothers of Robert. (four brothers did wind up in AR) John Wesley wound up in Dallas Co., AR. These last two brothers probably migrated some time after 1835, John Wesley after 1850. Other widowed members of the Stell family did come with the first three settlers.
As far as the George Stell and John Stell as the original immigrant ancestor, I am still searching for the right answer, One thing for certain is that the John Stell mentioned in Birdie's letters who bought land in 1739 next to George is definitely NOT the John Stell who married Susannah Malone! The John that married Susannah was born in 1743/44. It could have been John father of John. That is one of the lineages I have seen several times. If that is true, then George and John Sr. could have been brothers.
Also, the Stell from South Dakota who came to see Birdie was Kermit Stell. He and Al Stell of VA supplied the 100 plus pages of Stell lineages for the book "Malone and Allied Families" second edition by Randolph A. Malone. That meticulously researched book also states that a will for Robert Malone in Northampton Co., NC in 1777 left 8 pounds "to my daughter, Susannah Stell". That would seem to lock him in solidly as Susannah's father.