The brain rot was a joke at people calling her having a hamster brain or whatever.
I’m going to put this under spoiler tags as it’s less vague.
spoilers:end of spoilers
We don’t know how long she’s been reading the book but I think the clues are there. Her fingers are black and tarnished, showing she’s been reading the Darkhold for quite some time. Second: While it is a “facade”…. The orchard growing, showing time has past since there was none at the end of the WV. I assume the Darkhold could be tempting her and corrupting her mind, and knows what she wants and uses it against her.
Trauma and grief effect people differently. How you handle grief is different than how I would. How I would, it would be different than someone else. On top of all that grief and trauma, all happening throughout her life, and some of it being back to back (don’t forget she got dusted and couldn’t even mourn till she came back).on top of all of that, reading the Darkhold would corrupt and manipulate emotions and even possibly prevent the brain from making proper, rational decisions.
A real world example, someone with hypoglycemia would make sense and better decisions right away but the more you creep down in numbers, the more the irrational thoughts start to not seem so irrational. Your grip on reality isn’t as strong as it was 20-30 minutes before. It takes something rather big to snap you out of it. spoilers:end of spoilers
I’m not comparing Wanda’s deadly decisions with it, just stating that the more immersed in something troublesome you become, the more things that don’t make sense, start to somewhat make sense. I speak from experience lol.