There is a three volume set of 'Trevelyan papers' that might help you sort things out. (though some of the documents are in Latin) and cross check some of the dates and wives with those from secondary sources.
part 1 has inquisitions post mortem on John T Esq 1493, Sir John 1522 and John T Esq in 1547
part 11 contains 2 John Trevelyan wills dated 1550 and 1558 plus some marriage contracts
The Vic County History says 'From John Trevelyan (d. 1492) the manor descended in the direct male line through Sir John (d. 1521), John (d. 1546), (fn. 57) John (d. 1563), and John (d. 1577) to John Trevelyan (d. 1623). (fn. 58) The last named was succeeded by his grandson George Trevelyan, a minor' http://www.british-history.ac.uk/vch/som/vol5/pp111-120
History and Antiquities of the County of Somerset, Vol 3 page 539 has a brief pedigree. (it's viewable on google books but has a very long url)
"This John Trevelyan was descended from a family who had been for many ages of the most eminent rank among the gentlemen of that part of England, which dignity he by no means tarnished 27(1448/9) and 38 Henry VI.('1459/60) he was sheriff of Cornwall, and 31 Henry VI. (1452/53)was returned one of the knights for Cornwall in the parliament held at Reading. 7 Henry VII(1491/2). he was created a knight of the Bath at the marriage of Prince Arthur. He died 8 Henry VII.(1492/3) leaving issue by the above mentioned Elizabeth(Whalesborough) his wife, John his heir and successor at Nettlecombe , Thomas, George, chaplin to King Henry VIII. and Humphrey.
John his eldest son married Jane daughter of Sir John Halywell, knt. an eminent partizan of the House of Lancastor and died 13 Henry VIII.(1521/2) leaving issue another John, who married Avice, daughter and heir of Nicholas Cockworthy, of Great Yarnscombe in the county of Devon, descendants and heirs of the noble families of Champeron and Valletorte, and by them of Edmund earl of Cornwall. By the said Avice he had several children, of whom John the eldest married Maud daughter of Giles Hill, of Honiton esq. and was father of a fifth John Trevelyan, who married Wilmot daughter of John Harris, sergeant at law, of Hayne in the county of Devon.
John Trevelyan, son and heir of the last-mentioned John, married, in 1561, Urith daughter of Sir John Chichestor, of Raleigh in the county of Devon, knt. This John rebuilt the house at Nettlecombe, and 16 James I. was high-sheriff of the county.(1618/19)
He died in 1623, and was succeeded by his son and heir
John Trevelyan, the seventh of that name….." (pleased to see he was followed by a George.NB I inserted the dates in brackets using a regnal year calculator )
I noticed I haven't seen a single birth year, only deaths. I wonder where the birth years you have came from.