these are the timeline I clould get:
1. the queen spider loitering at the mc's house, a report abt missing professor at the same day (maybe slacking to look for his pet spider)
2. days passing, the prof proly managed to retrieve the spider and brought it to the girl's house, spider fled from the cage to her room, it bit the girl or anything to transfer all its power and memories to the girl (perhaps bc its instinct told it was in danger knowing all the scientists in shonen manga are like orochimaru), the girl started dreaming strange things, and the spider either dead or turned into regular spider idc
3. she was eaten half and her power activated that time, she heal herself and her lower half bc she could, report abt the remnants of a girl body were found later, it might just dead skin remnants as she was shedding her skin for the first time, not the cleanest one (she later shed her skin for beautification from time to time judging by how the mc found girls hair in his house), and she's actually been living under the mc's bed by the time the broadcast reported about a missing girl
4. now the girl has her personality mixed with the spider and she still couldn't control the switch