Don't be ridiculous. I don't have to "master" a spell to monopolize it. I could enter 500 Death spells right now, and never learn a single one of them, or even bother putting any points in the Death base. But once I've designed the spells, no one else can ever master them, either, because "I own the patents, bizatch!".
So, what is there in the system that prevents someone from doing something like this?
The basic misunderstanding here seems to be that if you invent the spell, no one else can ever discover it. That's not true and I didn't say that. What I did say, and will stick with is that there won't be duplicate spells in the database. I don't want to have 20 different Fireballs where the only difference between them is their name and the flame colour.
There will also be different methods of gaining spells than the "spell creation" interface. I want to leave that even though I don't want to have much invention, because it allows you, the players, to be creative instead of having to rely on whatever spells the small GM team can come up with. But
invention is not intended to be a major source of spells for your character.
The main source should be other characters. Since new characters will start out with spells that the older ones almost certainly already know, trading is not much of an option. But this is one way to get apprentices, favours, debts, etc. - which gives us stuff for roleplays, intrigue and conflict.
There will also be spells you can find through roleplaying. If you discover an old mystical library, the GMs may reward you with a few formulas.
Councils as a source of spell sharing have already been mentioned.
So please don't stick to spell invention as a source of spells. That's not what it's for. That is why there is no guarantee that you automatically learn the spells you invented - and also no guarantee that we won't be giving it to someone else.
