For those interested in when the Attic and Sekai (plus dress) was mentioned, it's Chapter 2's Ami 'Outcry of the Hunted Hare' Event. It's also when Ami had a nightmare involving Sekai:
I'd also think Ami talking about cleaning the Attic in her Chapter 1 'Harem Tutorial' Event (the first Ami Event), then later having Sekai's shadow show up behind her in her Chapter 1 'Cute Girls and Stuff' Event:
Are related. (This Event also leads to her becoming a Maid which is a Divergence to the normal timeline according to Maya Prime in Chapter 1 Ami's 'Divergence' Event)
In theory: Ami cleans Attic. Finds Sekai things/unleashes something. Starts having bad dreams and something like Sekai following her. Ends up having trouble sleeping as mentioned in Chapter 2's 'The First Signs of Fraying Threads' Main Event:
Which might be why she had a fever in the Chapter 1 'Little Girl' Main Event, and ever since, she's been getting worse and worse while the voices mentioned in her Chapter 2 'Living' Event:
Got worse and worse, along with the dreams, etc, and eventually...well, she loses her shit (after apparently getting murdered), breaks, and seems to just be doing what the voices tell her now: