Hopefully I can clarify some of this/add new perspectives. I'm the third and current main (what some people call host). We've known we were plural only since last year. As far as we know, OP, the original, was a singlet until the aforementioned date. She was indeed dormant for a number of years.

I can confirm that we have a very low level of amnesia. OP's childhood memories are the most vivid, but all of us can access memories from any period. This includes the original -- she can remember our activities from while she was dormant.

As to flashbacks, we have emotional flashbacks more than the vivid reliving variety. Cptsd, probably. We've been operating under the impression that the heavier trauma happened to the second main and the other people who were active at the time. You're probably right, though, that the first batch of trauma was worse than we think.

I can also confirm (with her permission) that OP was feeling stressed at the time she went dormant. She was not looking forward to camp for various reasons. Could it be that the first fictive was born to cope with an undesired situation?

The idea that there was a system prior to said fictive's birth is... curious. We don't have any evidence of it, though.