WHA Manga Spoilers and Theories:
We know the Brimmed Hats are infecting people with the Silverwood Tree parasite. Ininia has many of these seeds and is really cavalier with handing them out, and she might not be the only one. But at the same time, it seems like they actively don't want these people to actually turn, which means they need them for something else. The intentional human incubation of these seeds presumably started with Qifrey well over a decade ago, presumably as a test, and it must have been successful if they're scaling up the operation.
What we know of Qifrey's time with the Brims is limited, but we know that one of Qifrey's eyes was taken and replaced with the Silverwood Tree seed. His eye was then used by someone in the Brimmed Hats. It's pretty clearly alluded to that the person responsible is Iguin and it is also implied that Iguin did a similar spell that Sasarin did to transmute his body into looking like Qifrey, using his eye, because we know Qifrey is specifically looking for a Brim that has his countenance. Of the brimmed hats we have met, we can clearly see their faces except for Iguin's.
We have also seen Iguin do blood magic and cast teleportation spells without drawing seals. We assume it is forbidden magic, but maybe it isn't drawn or tattooed... I think the Silverwood Tree infection does something to the blood that infuses the inherently magical properties of the woodcruor ink into the person, making a physical, biological distinction between a human and a witch, where right now there isn't one. And it would also enable the witch to cast without drawing.
We have seen that Qifrey can do HUGE works of magic using proportionally small seals. I think this has everything to do with the magic ink circulating with his blood. I think that him drawing at all may be a performance because we know there are limitations to what palm quire spells can do - and the seal Qifrey drew to cast that water dragon was maybe the size of a dinner plate, yet it affected the whole area of the fake town. It's giving the same Staircase River scaling issue, which we know is directly correlated with Iguin's blood also.
So, I think that Iguin took Qifrey's eye to use it as a kind of vaccine-like measure so that he isn't directly infected by the seed, or with it he gained an immunity to the seed taking root, where he can still access the benefits of magic blood. This goes hand in hand with the whole Pointed Cap's vs Medical Science issue - the Brims don't care about that separation of church and state, so bringing medical theory into their spellcrafting makes sense. It is Iguin's mission to open magic back up, in terms of reversing the purity standards the Pointed Cap's enforce, as well as broaden accessibility and give magic back to the people (for better or worse). Being able to give people 'the gift of magic' without enduring the training, ethics lessons, or needing necessary technique to draw seals, seems right in line with that goal.
I think Iguin may even be working with King Dean already. We know Dean wants magic for himself, and he seems to be doing medical experimentation. If the way to bring laypeople magic is to transplant (or graft) living, human-Silverwood tissues/organs, then having someone well versed in the surgical arts would be necessary for Iguin. And I found it rather suspicious that the Western Medical Spire is so far away from the Castle's anti-magic field. If the point of the field is to protect royalty from magical harm, you would think with how much time the King spends with his medical practice, that said practice would be relocated to the grounds. But, if it was, then the magic transplants might not work at all? (Although, I am also considering the fact that Custas was able to bloom directly in the castle, which has its own ramifications. I guess the woodcruor only has magical properties upon death/withering, so that may be why??)
Anyway, that is all I've got for now 🤔