So, Jun has encountered Souseiseki in the Sea of Unconsciousness when he had mental breakdown in the 5th volume.
It made me wonder - what happens with dolls when they die? Do they meet each other or are completely lost?

Shinku says in vol.1 that when a doll ummm "dies", it doesn't completely disappear, but it goes somewhere else. Maybe the Sea is that "somewhere else" ? Also in the anime, Shinku says that even if Sou is gone, they will meet her again. So, I don't completely think that there's an afterlife or something.
And, in vol.7, Sou is missing form her box, and Sui meets her again in vol.8 (though I think this had something to do with Kira, since a rose suddenly appeared from Sou's eye...)

The nine seconds before white is an air pocket in the Sea of Unconciousness
Maybe they get trapped there?
Not really sure, but they seem to loose their memory...

I think that Kira somehow revived her without Rosa Mystica to trap Suiseiseki. It's pretty interesting as she seems to have enough free will to tell Suiseiseki to run away.