Reference work coverage of 'off'n'
Harold Wentworth, American Dialect Dictionary (1944) does not offer a clear theory of how off'n arose from its source expression but says that it means "off from" or "off of" and notes written occurrences published as early as 1866:
off'n, offen, prep. Off, off from, off of. Cf. outen [identified elsewhere in the dictionary as meaning "out of" and dating to at least 1870 (with a first citation againfrom Sidney Lanier].
[Earliest three cited examples:] 1866 G[eorgi]a. & he picked all the rocks froim off'n the groun'. [Sidney] Lanier 'Thar's More in the Man'.' 1870 G[eorgi]a. I got down off'n my load. Lanier '9 from 8.' 1871(1852) s.Ind[iana], s.Ill[inois], s.Ohio knocked the hind-sights offen that air . . lawyer. [Edward] Eggleston [The] Hoos[ier] Schoolm[aster] 268
Wentworth also notes early instances from Tennessee (1881), Vermont (1892), Mississippi (1893, Texas (1897), southeast Virginia (1899), Maine (1900), southeast Missouri (1903), northwest Arkansas (1906), eastern Alabama/western Georgia (1908), Nebraska (1911), Wyoming (1912), New England (1916), and multiple states from Illinois to Iowa to Kansas, Kentucky to New York (1917).
Another term of similar construction that Wentworth notes is abouten, with examples from 1931 (freom Florida and the southern mountains of the U.S.) and 1941 (from Mississippi, in Men Working, by William Faulkner).
Dictionary of American Regional English volume 3 (1996) pushes the earliest citation for outen back to 1867 and the earliest citation for offen to 1841:
offen prep Also sp ofen, off on, of(f')n, pronc-sp off'm {Chiefly Scotsa, nEngl dial; see SND affin, EDD off prep. 2.(14)} chiefly among speakers woith little formal educ[ation] Cf. -en
suff3; somewhat old-fash[ioned]
Off of; off.
[[Earliest three cited examples:] 1841 Spirit of the Times 27 Mar [18]42 A[rkansas], It took two men to hold Sam off on him, for that ar hat cost Sam a mortal sight of Peltry. 1867 Harris Sut Lovingood Yarns 34 T[ennessee], I hearn a nise like tarin a shingle ruff ofen a hous' at wun rake. 1869 [the instance from Sidney Lanier's poem 'Thar's More in the Man," noted in Wentworth as dating to 1866].
Early instances of 'off'n', 'offn', and 'offen' in the wild
A Google Books search turns up two fairly early instances of the spelling offn (with no apostrophe), both of which appeared in issues of the same Virginia publication. From "The Letters of Mozis Addumds to Billy Ivvins: Fourth Letter" in the [Richmond, Virginia] Southern Literary Messenger (May 1858):
I seas, Billy, I'm libel to run offn the trac is a injine on the Sowthsied wrailrode , as I hassint tole you uv nuthin hardly I begun to tell you uv. But wun thing mo I must knarate year I quits this heer epistul, which ar is follers: ...
And from William Burwell, Exile and Empire, serialized in the [Richmond, Virginia] Southern and Western Literary Messenger (October 1861):
"Well, Cap.," observed Vince, who, under the tacit suspension of discipline, had approached his superior, "the cannon does roar dreadful. I expected to see that little fort knocked into the ar as easy as I could knock a squirril offn a tree. I'm afeard they must be all killed."
Somewhat earlier is this instance from "How Mick Shouter Came Very Near 'Walloping' Arch Coony: A Yazoo [Mississippi] Sketch," in Southern and South-western Sketches: Fun, Sentiment and Adventure (Richmond, Virginia, 1852):
So we kep er ridin' on an' ciussin on another worse than two Choctaw Injuns, an' torectly we cum to the ferry boat whar we had to cross the river. Soon as we got thar, Arch he hopped down off'n his ole hoss, an commence schukin' his self fur er fight, an I jumped down two.
And from Emma Southworth, The Lost Heiress (Philadelphia, 1854):
"Business? Is you turned a sinkin', infunnelly fool, in your ole days? Jess like business wur gwine to fetch him here almos' ebery mont'. He usen to come more 'an once a year; now, bless patience, ef he don't trot ober here almos' ebery mont' dat roll ober iour head. Better not be arter our Miss Ellen. I make him fly off'n dis yer plantatshum faster 'an ebber he comed on to it! I aint agoin' to 'low no marster put ober me, at my time o' life, 'deed aint I. ...
But older than any U.S. match I have found is this one from "Sam Pogson's Jealousy, or Major Bunce: A Chapter from Sam Pogson's Unpublished Biography," in Tait's Edinburgh Magazine (March 1836):
'What!' says I, 'then are yo' sartin as yo've no leaning whativer to th' Major?' 'No, sartinly,' says she ; 'and yo' ought to be downright ashamed of yourself, Sam, iver tio let onythink o' th' sort get into your head.' That melted mae a bit ; so I goes up to her, and gies her a great big kiss, as knocked her off'n her stool ; and at th' same time, says I, 'Come then, Kitty, let's mak' it all up, and I'll niver think noat o' th' sort no more—that I won't.'
And from "Miscellaneous Conversations: Matrimonial Discussion," in Abel Bywater, The Sheffield Dialect (1838):
O avver, o shawnt wunder ift oud mare dus'nt crush thee to deeath sum neet, if thah du'nt ger a woif to knock her offen thee. But after all this, dus'nt thah kno at this world's made up a contraries ; an at whether a chap be single or dubble, he's shure to meet we a gud deeal a trubble.
Conclusions
Offen/off'n/offn used in the sense of "off" or "off of" appears in U.S. writings from at least as early as the 1852, and DARE cites an instance of off on used in Arkansas in a similar way from 1842.
To my surprise, slightly older instances of offen with the meaning 'off' or 'off of' appear in Scottish and English publications from 1836 and 1838.
I can't tell whether off'n/offen originated in Britain and migrated to the U.S. or appeared independently in both places, but it is certainly not a unique form of extension (or contraction, as the case may be), as we see with the similarly framed words outen and abouten.
In the U.S., early instances of off'n seem to be concentrated in the South—especially near Richmond, Virginia—but by 1920 the word had been recorded in states from Wyoming to Maine.
'nand be simplified to "fell off the stool" Is there any additional information at all ?