Wild theories
1. The tulip bulbs
As I re-read, I wondered about the story of いとこ stumbling upon these discarded tulip bulbs. I wondered whether (perhaps like a dark fairy tale or Stephen King story) they contained within them the origin of the changes - the infestation of the dormitory, the disappearance of the students…
2. 先生
This is a totally wild supposition, but I had one thought that perhaps 先生 was the child of 姉 in the first story, and his amelia (apologies for the gratuitous bit of medical terminology (absent limbs)) were the consequence of 染色体の破壊
Some of his enjoyment of the sweet お土産 seems reminiscent of 姉’s pregnancy altered appetite…