How early does that change in a system from Non-Trauma to Trauma have to happen? And is is it known yet, if a Fictive can be ‘Grafted’ to a Core as it’s host during said near death experience?

I think the cut off for trauma creating a system is around 8 years old. As for a fictive being grafted onto a core, none of that makes any sense in an accepted medical/scientific context, but in terms of an endogenic system it sounds entirely possible that a near death experience would cause that.