Your Perry Green Magness still has me mystified, too. I'll share what I have, in the hope that it might tie into something you've got.
Yes, the affidavit I mentioned actually was the power of attorney. And yes, I do have a copy of the handwritten record, which I'll be glad to send you by snail mail if you want it. Here's the full text:
"Know all men by these presents that I Perrygreen Magness of the County Warren and State of Tennessee for Divers good causes and considerations have made and by these presents do make constitute and appoint Frances Alexander my trusty friend of the County of Rutherford and State of North Carolina my true lawful sole and complete agent and attorney in fact for me & in my name room & behalf to sue for demand ask and receive any moneys lands Negroes or any other properties which I might or may be intitled to of the Estate of William Magness Late Deceased in Lincoln County North Carolina & also to make titles to any lands which might or may be requested or necessary in the Division of the Landed Estate of said William Magness & to give receipts where required in as full & ample maner as though I was present myself & I do hereby for myself and my heirs undertake and bind myself to abide by ratify and confirm all and singular the acts and doings of my said attorney in fact in and about the Estate and premises in the same ample & complete manner as if done by myself in proper person given under my hand & seal this 4th day of September 1816." This was witnessed by Wm. Hicks (X) and John Fisher and was proved in Lincoln County court in the April term, 1819.
I have the same question you do as to whether your Perry Green was the son of Peregrine and Mary. If the Mary in Peregrine's 1800 will was the same Mary he was married to in 1760, then obviously she wasn't the mother of a child born in 1796. If she was a second wife, then she's still a possibility. But let me quote you from a document prepared by another of your cousins, David Hennessee, which turned up in the library in Warren County:
"George Magness born 1768, North Carolina, married Mary "Polly" Durham, born 1776, North Carolina, [who] died 1830-40, DeKalb Co. TN. . . . Before he was twenty years old, he was in trouble with the courts, and in 1794, he got in real trouble. In April of that year he was found guilty of petty larceny, his motion for appeal was overruled, and the court ordered [that he receive ten lashes at the public whipping post]. . . . in October 1794, [John Magness] made a 200 pound bond to indemnify the county from the maintenance of George's base born child born of Polly Durham. On May 20, 1795, George Magness sold to William Earles 50 acres on Hickory Creek, which the State of North Carolina had granted him two years earlier. He was thus left with no property to pay court costs or to make bond, he was held in jail from 26 June 1795 to 16 September 1795. He was released after being tried and found not guilty. . . . apparently he married Polly Durham shortly after his release from jail. Polly was the daughter of Achilles Durham and his wife Mary Cates, and was a source of much grief to them. Polly's parents were dedicated church members, and her behavior disappointed them greatly. However, her family stuck by her, especially her sister Sarah, who married Abraham Cantrell. George Magness, however, did not stick by her. Their first child, Sarah (Sally), was born in 1794, before they married. Their second child, Perry Green Magness, was born May 23, 1796, apparently after they married. Perry Green Magness eventually was able to establish himself as "the son and only heir-at-law of George Magness." However, this was accomplished only with considerable difficulty after eight years of litigation. Sally was not considered an heir because illegitimate children were prevented by law from sharing their parents' estates. . . . George Magness apparently did not live with Polly long after their son was born . . . Polly was left in Carolina with two children and little else. She apparently made her home with or near her sister Sarah and Sarah's husband Abraham Cantrell and moved with them about 1808 to Warren (now DeKalb) County, Tennessee. About 1827 Polly married a widower, Benjamin Cantrell, an uncle of Abraham. She died a few years later, between 1830 and 1840."
I don't know about you, but this all sounds pretty convincing to me. The only problem is that David assumes throughout his account that George Magness, father of your Perry Green, was the son of Peregrine, and I still have some unanswered questions about that (more below). The parts I've left out are the parts that refer to this relationship, because I thought we might be able to look at the data more clearly without the automatic assumption of the father-son relationship.
I've seen a copy of the court records about George's conviction for larceny. Peregrine's eldest son William was one of the bondsmen in this case. For some reason I didn't turn up the bastardy bond in 1794 or the jail sentence in 1795, and I haven't seen anything on the supposed battle to prove that Perry Green of Warren County was the son of George Magness, but David seems to have some solid research to back him up. John (Jonathan) Magness, who appears in the bastardy case, was also a son of Peregrine.
If it weren't for one document and one census record, I'd agree with David that the George Magness who married Polly Durham was also a son of Peregrine. But, on May 24, 1817, Perrygreen Magness, son of George Magness deceased, of Orange County, Indiana, filed papers in Rutherford County appointing Berryman Hicks his attorney in the estate of William Magness. This document was filed in the Orange County court, two years before your Perry Green Magness of Warren County, Tennessee, appointed Frances Alexander as his attorney in the same estate. In 1820 there was a Perry G. Magnus in the census record for Jefferson County, Indiana, just a few counties over from Orange County, and the census record shows that he was already over 45 (and thus born before 1775). He was also in the 1830 census of Jefferson County but not the 1840 one.
There's a possible wild card in all this, though. By 1840, Benjamin Magness's son Perry Green, who was born about 1767, was in Berrien County, Michigan. I have no Rutherford County census record for him after 1800, so it's entirely possible that he was the Perry G. Magnus who had been in Indiana in 1830 and 1840.
Even as I write this, I'm wondering if it's possible that your Perry Green Magness could have moved briefly to Indiana and then back to Tennessee. Orange County is fairly close to the Kentucky border and almost due north of Warren County, where Peregrine's sons George and Joseph had both been living at the time of their father's death in 1800. If the story of George Magness' abandonment of his family is true, I can't imagine why his son would have followed him to Kentucky and maybe on to Indiana, but this is certainly a possibility. The fact that the attorney appointed in the first document was named Berryman Hicks, and your Perry Green's sister Sally is supposed to have married William Hicks in 1812, also lends some support to this possibility.
As for Sarah Hamrick -- heaven only knows! Court records show that Peregrine's wife was named Mary in 1760, and that his wife was also named Mary in 1800. Yet Benjamin Magness' descendants have always said that Benjamin's mother was Sarah Hamrick and that she and Peregrine were also the parents of nine other sons and one daughter -- named Sarah. I do know that the Hamricks and Magnesses seem to have been connected after their arrival in Rutherford County, but I haven't found any evidence of a connection beforehand.
Regarding the possibility that it was actually Peregrine III who married Sarah Hamrick, I've seen the same nonsense on the web that you have. And I laughed just about as hard. If I had to guess, I'd say that somebody came across a statement that said "Peregrine Jr." was married to Sarah Hamrick and assumed this meant Peregrine III, not knowing that there was a Peregrine Sr. who was a blacksmith back in Maryland and Virginia. Actually, I can't imagine how the family tree we've seen ever got put together in the first place, because most genealogical software would slap your hand if you tried to tell it that a firstborn son (William, 1747) was 24 years older than his mother (Sarah, 1771)!
I meant to leave you a short reply. I always mean to leave short replies, but I seldom do. I hope this has given you some food for thought. Please let me know what you think.