Ok I actually have thoughts (TM) about this. It boils down to the fact that, while they may have individual goals and plans, the brimhats as a group are basically a loose anarchist collective. Their purpose is to subvert the current status quo of witch society. Since that can happen a lot of different ways, they tend to succeed even if their specific plots are foiled. (Huge manga spoilers warning, of course).
My partner, when reading the series, referred to these as "stick of dynamite" moments, as in another stick of dynamite has been added to the pile and he's waiting for the Boom. Qifrey is erasing people's memories and could be caught? Stick of dynamite. Euni was transformed and now both he and Alaira have to flee? Stick of dynamite. Tartah has secretly started learning apothecary and medicine? Stick of dynamite. Tetia is forming a friendship with the local prince? Throw on another stick of dynamite.
On paper, the pointed hat witches keep succeeding and stopping brimhat plans. But the actual victory for the brimhats is in the collateral damage and who ends up questioning or subverting the underlying system. In transforming Euni, all of Qifrey's atelier became complacent in using or hiding forbidden magic. In wiping Galga's memory, Ininia causes Atwert to question the principles and set himself against the Knight Moralis. Beldaruit's clever solution of making an exception and creating the counterclock device contributed directly to the political disaster of having the King bow to him.
Take the leech, for example. On paper, the witches succeeded in defeating it without revealing the secret of magic, they used the counterclock seal in a controlled device, and all the townsfolk cheer at the end about how great witches are. But how many people were saved by Custas, or saw him save others? How many people are going to wonder why witches didn't use 'healing magic' like the counterclock seal before now? How many are going to wonder why witches don't involve themselves more in natural disasters? It's why Easthies snaps- he can see that, even with excuses and official sanction, making all these exceptions undermines the overall system, and right or wrong, he believes in the system. And causing a schism in the Knights Moralis is a huge victory for the brimhats! Chapter 87 straight-up tells the audience that the balance of power was permanently altered that night, even though the pointed-hat witches "won"/succeeded.
In chapter 88, there are two pages of Iguin saying "They've seen through the lie, beheld the true might of a power they've long been told is not a panacea to all woes. Soon, once whispered prayers...will rise into a chorus of desperate pleas...then transform into a symphony of furious cries". The art of these pages shows every character that has, one way or another, been subverted and now has doubts about the current system, or is in conflict on it:
Jujy and Hiehart? Saw Custas saving lives with forbidden magic.
Easthies? Coup in what he sees as the corrupted knights.
Lagrah/Beldaruit? Have lost the careful equality between witches and royalty.
Riliphin and Eoleo? Developing a potentially subversive friendship.
Luluci? Has bent the rules as the Wise In Principles
The other knights? Torn/conflicted over Galgah, Easthies, and Luluci.
Galgah and Atwert? Set on opposing the knights and finding a way to save him.
Tartah and Custas? Straight-up walked away with the brimhats.
Ininia? A brimhat but having her own conflict over Galgah being kind to her.
All of Qifrey's atelier, adults and apprentices alike? Have seen and done/contributed to all of the above.
The brimhats are winning.
So yes, say Coco wasn't saved by Qifrey. How many people in her village would wonder and investigate the sudden appearing circle of stone before the Knights could show up to wipe everyone? Say Coco just never copied that spell. She would have kept practicing on her own, potentially telling others (her mother, other kids in her village) and spreading the secret of magic. How far could it get? Could the knights be sure they wiped *everyone*? If Coco never figured out that magic was art at all, would she have passed the book on, given it to a friend or (far in the future) her own kids who might have copied or traced the designs? How many other kids got books like that, ticking time bombs waiting to explode? The brimhats don't need any one specific thing to happen, they need enough cracks to form for the whole rigid system to collapse on itself.
