Johnnie,
Happy Thanksgiving Day to you and all my Shelby and Polk kinsmen!!
To answer your questions:
I find no evidence/data that ‘my’ Eleanor Shelby was the d/o Rees Shelby & Mary Blair. If so, I believe it would have been known by now, especially with all the research that’s been done throughout many years.

Yes, I know that Capt. John Polk (husband of Eleanor Shelby) was an Indian Agent of the Catawba Indians during the Revolution, also serving in other regiments, which I will list below. I will pass along the data that I have and I believe it to be correct. Sentences included in (( are my own words. As follows:

John POLK, Capt., born abt. 1739 in Cumberland Co., PA., died abt. 1785 in York Co., SC.
John was s/o William Polk of Somerset Co., Maryland & who died in Anson Co., NC. And Margaret Taylor born in Philadelphia, PA. Margaret died in Mecklenburg CO., NC.
Capt. John POLK, m. Eleanor SHELBY (sometimes referred to as Elinora or Elloner) married 02 Oct. 1758 in Anson Co., NC. Eleanor was born in 1730 in Tregaron, South Wales, died in 1803 in York Co., South Carolina, she was d/o Evan Shelby & Catherine Morgan.

John Polk, Capt. was born Abt 1739, in Cumberland Co., near the site of present day Carlisle, PA. He came with his parents to Mecklenburg Co., NC. About 1750, and in a deed of 1763, on file at Charlotte, he is styled "a planter."
His wife "Elloner" joined with him in signing a deed in the same county in 1764.
On 7 June 1766 he appears as a member of the Clear Creek Company of militia, commanded by Capt. Adam Alexander, in which his older brother, Charles was a lieutenant. He was an officer in Col. Francis Locke's regiment.
Capt. John Polk died probably in early 1785, as on 9 Sept. of that year the Assembly of North Carolina issued Land Warrant No. 2149 "to the heirs of John Polk" for "1000 acres of land within the limits of the land reserved by law for the officers and soldiers of the Continental line of this state." He married Eleanor Shelby, 02 Oct. 1758.
Ref: [1] "Polks of North Carolina and Tennessee", by Mrs. Angelloti.
[2] "Polk Family and Kinsmen".

Children of John POLK and Eleanor SHELBY:
#1. Shelby POLK.
#2. Eleanor POLK, m. John THOMPSON.
#3. William POLK, m. Elizabeth McLanahan.
#4. Charles POLK, he moved from Mecklenburg Co., NC. To Western Tennessee and then to San Augustine Co., TX. Abt. 1839.
In a pension declaration on file in Washington made by him under the Act of Congress of 1832, he states that he served at various times during the Revolution in North Carolina Troops, under Capt. James JACK; Capt. John POLK, his father; Charles POLK, his uncle; Oliver WILEY and FLETCHER and Colonels Adam ALEXANDER and Col. Thomas POLK his uncle. And that he served in a mounted spy company
for 5 months and 28 days in 1814. He is called in many old family records, "Civil Charley Polk," probably to distinguish him from his cousin, Charles Polk son of Brig. Gen. Thomas Polk, who was called "Devil Charley."
Ref: "Polks of NC & TN" Page 19.
#5. John POLK, b. 1767 in Mecklenburg CO., NC., d. 24 May 1845 in Maury Co., TN. m. (1) Elizabeth ALDERSON, before 1790 in the Greenbrier Dist. Of W.VA. Elizabeth was born abt. 1766 and died 24 Nov 1829 in Maury Co., TN.
The tomb of John Polk and his first wife, Elizabeth still stands on the land of the homestead on CarterÂ’s Creek. On one side are the words; "In memory of John Polk, died 24 May 1845 aged 78." On the other side is the inscription; "In memory of Elizabeth Polk, died 24 Nov 1829 aged 63.
John & Elizabeth Polk had 7 children. John Polk m. (2) Mrs. Rebecca BRIGGS, 12 Jan 1835 in Maury Co., TN.
#6.** Taylor POLK, Sr., b. abt 1780. Where Taylor Polk was born is not certain, but it is probable that he was born in Mecklenburg Co., NC. or immediately across the state line in York Co., SC., many Polk families settled in that area and moved back and forth across the state boundary. Taylor Polk died abt. 1838 in Arkansas Territory. The 1790 Census of York Co., SC. shows TaylorÂ’s father, John Polk as residing in York County with one free white male under 16 (Taylor) and one female (TaylorÂ’s mother, Eleanor) in the household.
The 1800 Census of York Co., SC., page 629, shows Taylor Polk living in his own household.
It shows John and Charles Polk on page 629 of the census (this Charles was probably TaylorÂ’s older brother) Taylor was a big man, boasting that he "stood six feet two in his stockings" at age seventeen.
Taylor Polk apparently lived in York County for several years after his marriage to Jency Walker in 1798. He was there on 06 May 1803, when he signed his motherÂ’s will as a witness. He was able to sign his name on the will, indicating that he had at least a rudimentary education.

The majority of the members of the Polk family in that area left for Tennessee around 1806. It seems reasonable that he may have gone to Tennessee with that group, settling in the area south of what is now Nashville, TN. in what was originally Davidson Co., TN., later, Maury Co., TN.

Sometime between 1806 and 1815, Taylor probably went to what was then the Louisiana Terr., now Arkansas. Oral family history states that Taylor and his family were in Arkansas by 1808. Taylor Polk, Sr. married (1) Jency WALKER, 21 Nov 1798, d/o Tandy Walker of VA. (2) Polly WARREN aft. 1814.
((I descend from Taylor Polk, Sr. and Jency Walker.))

Notes & References:
Eleanor (Shelby) Polk, thought by Emma Angolletti in her history of the Polk family, "Polks of North Carolina and Tennessee," to be the daughter of General Evan Shelby. Actually, she was the sister of General Evan Shelby. Eleanor was the daughter of Evan SHELBY and Catherine MORGAN.

Eleanor (Shelby) PolkÂ’s will, signed by her X on 06 May 1803 and probated 31 October 1803, is on file in York County, South Carolina, Will Book A, Page 129, Case No. 61, File No. 350.
The witnesses of the will includes her son, Taylor Polk, her daughter, Eleanor, and Wily Sanders. She appointed her friend, Robert Harris to serve as the executor of her estate. Other records pertaining to her estate can be found in Will Book B, Page 474 and Page 550 (or 551).

Cousin, Bill Polk of Kansas City, MO. has many of the records in his possession. I know this message is rather lengthy but maybe will help someone in their research.
Let us all work together to connect our ancestors!
Kind Regards,
Lola Withrow