Although Tobio's avatar Omnis can undo the heroines' deaths (except ones who are corrupted into demons) by using his ability, it seems that the heroines' deceased relative/family members don’t get resurrected, such as Seira's older sister, etc. It appears that Omnis can only alter events that occurred after he came on the scene, while the mentioned girls' personal losses occurred long before that.
Tobio's Mentor Mascot avatar Omnis resembles a black cat. This could be a reference to the Schrödinger's Cat theory, which is related to The Multiverse, one of the game's major plot points.
When Seira and Lilly met their ends, the demons that killed them had a face that resembled themselves. It is likely that other victims who get killed (or devoured, for normal bystanders) by (a) demon(s) don't see the faces of the magical girls but instead someone they love or care about, or worse themselves.
However, it's not clear if non-magical girls (read: NPCs) can be corrupted into demons after they're killed by demons. There is large possibility that only magical girls (even the normal heroines) can be slain and corrupted into demons.