I think the minigirl is the doll that plays an important role in whether Shion goes insane or not. Not sure about shadow Keiichi though. And no idea bout the cucumber.
I found out what the cucumber and eggplant are all about. The two vegetables are part of traditions during the annual Bon festival in Japan. Bon is a 3 day festival of the dead and the two vegetables represent animals that the dead use to travel to and from the world of the dead.
Cucumbers are horses (fast). Eggplants are cows (slow). The dead enter the world of the living riding on fast moving cucumbers and leave on slow moving eggplants to give as much time as possible in the world of the living. So, your family assembles horses and cows using vegetables and disposable chopsticks for legs. Then you take them to your family grave site for their use. This is all in addition to other traditions surrounding the Bon festival.
The cucumber and eggplant as vegetables seem most often associated with kawashiro_nitori and kamui_gakupo respectively. In their shouryouuma forms as above it makes sense that they'd be associated with spirits or people who follow traditions.
