This is a continuation of my Specifics of magical symbols and casting series of questions.
In the process of splitting my post up, I noticed my final question got a bit broad. So to specify how magic works in this world:
Magic in this world is omnipresent, yet largely like science in discovery process. It holds no consciousness or purpose, but acts like a program, "seeking" similar patterns. Runes are largely created in the "library" of magic by association.
For instance, a drawn flame in a cave to signify fire, used again and again over countless generations, created the association that this symbol means fire in the "Magical Library" that permeates all existence. In our world, this relates to a Deep Learning AI, except... there's no code, no point, no purpose. It simply, is.
This also opens up the possibility that, either in densely magical areas, gathering of like minds, basically permeating, society-wide philosophies can create deities, manifest purely-magic beings, or infect a town with fervor, inexplicably turning everyone within the region to the same fervor if they aren't strong of will. Ultimately, this component of nature that just is actually could gain sentience, however, due to the nature of the beast, it could never truly claim autonomy over the entirety of magic.
Magic exists regardless of belief in it - however, belief or mass fervor can and often will generate a certain outcome. Say, people feeling hopeless in a ravaged, pillaged, swamp town may always be surrounded by darkness. Sudden, widespread happiness won't make this disappear in an instant, as it is a magical abstract that has built up over the region that will only go away either in time, or if diminished and replaced by a fervor of emotion that would combine or overwrite it.
One caveat, however, is that you couldn't just get a bunch of random people together and say 'draw this over and over again,' as new functions of this "Magic Library" must draw from some sort of mass phenomena, where some emotion is in a state of mass hysteria among a region, or a belief isn't just skin-deep. The stronger and/or more universal a belief and/or symbol is, the wider the scope this magic can be reliably used in.
This question is most related to the difficulty of mastering magic manipulation
So, we've defined that the "Magic Library" can't be abused, you can't directly influence new spells and abstract ideas within it. However, there will always be those that try, and will find a way to, somehow, manipulate it to make a spell they desire.
How would someone master creating new runes, symbols, and spells for mass/their own use, with the sole intention of creating it?
Remember, the catch is that you can't just make some spell because you want to. New magical 'functions' are made solely by consistent association and widespread belief/feeling of something.
Individuals are able to, over years, have their own unique magical functions, whether its an aura, spell, influence, etc available only to them. Say, a hermit who does the same routine for decades, may find that just by initiating their routine, the objects and environment act out the scene, even if he stops, to a weak degree. Like, predictive reaction to what usually happens. So he may first notice it by noticing a plate that he's washing seems lighter than normal, which behind the scenes, is driven by the Magic Layer preparing and replicating his movements within the world. Again, like an AI that Learns. Also explains "Haunted" homes or locations.
Therefore, while that is a subconscious event, completely unintentional, how would a person try and succeed to get around the caveats of the System - it must be within a meaningfully short amount of time. Say, a year at most.