It is in canon that before the order of Hermes, magi would go around killing other magi for their spell-casting secrets.
This certainly makes it sound like inventing spells was a lot more difficult than hunting down another magus and robbing him of his spells (and probably life).
The Cult of Mercury had access to less than 30 great rituals.
Does it sound crazy to think that the main source of formulaic magic in pre-OoH times is that either your godly patron (Mercury, Diana, Pluto, et al) either taught you directly, or you had a good Twilight incident where you returned with knowledge of a new spell?
Well, every morning a spell-casting priest would pray to Mercury, and the god might see fit to send a spell or spells into the cleric's mind; not necessarily the ones the cleric asked for. If the spell was cast...
I am given to understand that Magical Warping apparently is a problem that occurs to every magical tradition, but tends to express itself differently. Eventually each of these different magical traditions would hit a hard limit that Hermetics call Final Twilight, so for lack of a better understanding I tend to use the terms "Twilight" and "magical warping" somewhat interchangeably.
It appears the Founder Criamon is responsible for Hermetics having a consistent set of responses/defenses to magical warping/Twilight.
However Criamon came from what is considered a Mercurian background, so presumably his consistent set of responses was derived and distilled from the history of magical warping experiences his Mercurian brethren and forebears recorded.
Hence I suspect that amongst the various magical warping responses that mercurian wizards suffered, a subset would have been what Hermetics would recognise as Twilight. Thus I assume that some mercurian wizards could have had a (quasi-)Twilight experience and brought back knowledge of a new spell.
I don't recall reading anything suggesting that Mercurians had a magical theory that allowed for new spell creation.