First off, omg thank you for reading and thinking about this and taking the time to give me such thorough questions and thoughts!! <3

The changes in mental state are almost purely in stage 2, and fade off at stage 3 as overall sensation starts to go and a bit less dissolving-you-alive is going on. I left the rest as "unpredictable behaviors" simply because the only things affecting the mental state of the infected after stage 2 is due to either how the host is coping with the situation (probably a lot of instances of aggression, depression and anxiety due to fear or trauma, as well as all kinds of responses and dysmorphia from just having your body change in crazy ways you don't understand, not to mention responses to sociological issues as society fell), or due to very physical problems the fungus causes based on either location (e.g. some of the fungus is rooted close to the brain and causing problems there, akin to the problems brain tumors may cause) or based on mutations (e.g. mutations may affect the thyroid and cause changes in behavior because of the thyroid's handling hormones differently).

The mutations can physically empower or weaken any of those affected by it, whether or not you get something useful is just a roll of the dice, and whether or not it's perceived as a power or a weakness depends on the opinions of the person affected. Boneworm otherwise doesn't alter behavior and people are still much as they were before, mentally speaking (I generally want to avoid them being argued as inherently less-than-human or not human after mutation, and changes to a persons psychology does that much more so than changes to physical appearance), although they may adopt animalistic behaviors simply because they grow to identify with the attributes they've developed in some way.

I'll clean up the doc to be clearer about all that later.

I left boneworm's background vague for a variety of reasons:

• It's enigmatic for me, so I'm less inclined to write characters who understand it more than necessary (because I'm very inclined to do so when the information is there, aaaaand I'm very inclined to put that information there aaaaaugh)
• It's enigmatic for readers, so they can come up with their own theories and ideas based on things that they've seen and learned in the comic (which is also a cool way to mirror a lot of what I want to talk about in the story: everyone has incredibly variable ideas about whats going on and nobody really knows who's right. I'm interested in approaching the values and problems of ignorance and how it impacts our abilities to create, understand and cope in Defans Amis.)
• It's just not entirely necessary for the story, I generally want to avoid breaching ideas like "lets try to fix it" and mostly want to focus on "what do we do now that we're all scary and have mushrooms coming out of our fingers? We need to think about whats for dinner and how we can get our phones to work again, guys, I need to know if my aunt in Alabama is okay." End of the world infection stories often focus on going back and what was, and I kind of want to focus on moving forward and the coping mechanisms people use for that in this regard. By all means, people may be trying to come up with a cure/treatment, just not the people Alette and Beau are going to come across.

However, reading this and writing this out more or less made me decide to approach however I myself think of boneworm's functionality as it spread early on as also just another theory that is just as valid as any other theory, which may allow me some leeway as far as my first point goes. A lot of what I've thought about in regards to how it must have worked involve things like you mentioned, although my alternate for fungicides was simply the pervasiveness of boneworm; since it infects literally every macroscopic creature, basically all dead bugs need to be avoided and EVERYONE has to live on fungicides forever, not to mention how livestock and other important animals will be affected and impact the sustainability of human societies. I also really like the idea of the fungus trying to take down the host in response to treatments. People in the comic may bring up some of these ideas you've mentioned as their theories, though~ :3 (as well as magic and Jesus and whatever else they can think of, which also may or may not be true)