The first use of 'spliff' I could find was in the 27 April 1855 Lincolnshire Chronicle [paywall] where, in its singularity, it might well have been a mis- or deliberate re-spelling of 'spiff' (1862, OED), the shortened form of 'spiffy' ("smart, spruce", 1853, OED, in Letters of Dante Gabriel Rossetti; 'spiffy' with a broader sense of "decorated, spectacular" appears perhaps as early as the 1773 publication of Miscellaneous Poems, John Byrom, in an interpretation of one of Horace's Odes, and certainly as early as 1790 in a literal translation of Horace's Ode X):
...and in it was announced the fact that Mrs. Lyon was at Dover "as spliff as could be, and sporting a black veil."
The next use in the sense of "smartly dressed" was in the 27 August 1881 North Wales Chronicle [paywall]:
...she must have me sport (wear) my uniform to show me off to her pals respectable like, yer known; so I togs up spliff (dressed himself smartly), an' orf we goes like any toffs (well-dressed people) up to the nines (the height of fashion).
Although the sense of the 1855 and 1881 uses in UK newspapers is a bit different, they or similar uses may well have sponsored the adjectival use of 'spliff' as a neologistic advertising argot portmanteau-blend (the blended element is the long 'ee' from the end of 'spiffy') of 'splendid' and 'spiffy', as seen in a series of ads from the late 1800s in the Jamaican Kingston Gleaner [paywall].
For example, here is one such ad from the September 1889 Kingston Gleaner [paywall]:
To the Ladies.
Just opened up the most unique and amazingly Spliff Double Terai Hats for Ladies! Not a Lady should be without one. They are of the very best quality and fit to adorn the beautiful!
(OED: "terai, n. 2. transf. A wide-brimmed felt hat with double crown and special ventilation, worn in sub-tropical regions where the heat is not so intense as to necessitate the use of the sola topee or pith sun-helmet. More fully terai hat.")
Ads for hats and other dress goods using the 'spliff' coinage continue in (and only in) the Kingston Gleaner through the 1890s.
A first use of 'spliff' with reference to ganja appears in the Kingston Daily Gleaner in 1929 (May 17, paywall), in a news story titled "Herbert Breakenridge, the 'Axe' Murderer":
...The deceased said that he was
IN HIS "SPLIFF."
He did not know what spliff meant. The deceased held the accused and began to draw him meanwhile he brandished the knife in his hand, saying : We must live in unity with each other....
Deceased said: All these fellows around here form up together and I don't see why they should not
LIVE IN UNITY.
He did not hear deceased say: I in a spliff. He did not know the smell of ganja. The deceased was smoking at the time....
The deceased was smoking a ganja spliff. Deceased said: You fellows around here grow up together and must not have anything but live in unity. Deceased also said: I in a mi spliff. He smelt the ganja and told deceased to go away.
A shortened variant spelling of the portmanteau 'spliff', 'splif', shows up again in the blended sense of "splendid and spiffy" in an ad in the November 25, 1929 Kingston Daily Gleaner: "...Mexican Drawn Thread Tea Cloth, real Splif".
In the March 13, 1934 Kingston Gleaner, the sense implied is again "marijuana cigarette":
In regard to the ganja trade there is many a yokel with ganja growing in a callalu piece, and the weed is brought up to Kingston by the bagful. Spanish Town Road has many a clearing-house for ganja and ganja-smoking parties were at one time common in that region. "Pass the spliff along" is the ganja smoker's watchword until he gets into a frenzy and commits some murderous crime.
[A "callalu piece" is a plot of land used to grow culinary vegetables. Ed.]
Considering that the advertising argot adjectival use of 'spliff' in the sense of "splendid, spiffy" (itself possibly sponsored by the rare earlier UK use of 'spliff' in the sense of "smartly dressed, spruce") and the later early appearances of 'spliff' in the sense of "marijuana cigarette" are highly localized, that is, only found in the Kingston Gleaner, such uses are unlikely to be entirely disjoint.
Although there is no affirmative evidence to contradict the fanciful proposition that 'spliff' was first used to mean a cigarette containing a mixture of tobacco and marijuana, equally there is no affirmative evidence to suggest that the contents of a spliff were anything other than marijuana. In short, ad hoc origin stories aside, there is no reason to suppose that 'spliff', as originally used, meant anything other than "ganja cigarette" and, by extension, "ganja".