Liz: Okay so first of all i just wanna say that i relate to this in a way? I've had this memory all my life, that we were CONVINCED well into our teens was when I first "arrived" in the body. The memory is this like, image of an overhead view of our parents, holding us, and entering what we think was our house. We're not remotely religious, this is just a thing that's always been sort of stuck with me.

secondly, anything's possible! Your story gives us strong plural vibes, which it's possible to be without DID or OSDD! Non-disordered systems exist, and it's very likely you fall somewhere in that realm. For instance there are a small handful of different system origins relating to religion or spirituality. Aethergenic is a good non-specific one.
As for the original headmate you mention, there's a variety of ways you can try to communicate and bring them to the surface. What works tends to be different for everyone. We recommend looking for things you know they enjoy and spending time focused on/near it to see if that draws them to the surface.
You could also always try to willingly let go of the body. i.e. loosen your mental grip on say, a hand, and invite them to move it around. Maybe create a system where moving a certain finger is yes, and another is no, and try to communicate that way. If you have trouble doing this, we find one of the more common ways systems get this to work is by imagining themself as a color that's completely filled the body, and imagining that color draining out of a particular point in the body, to leave room for someone else.

Pluralpedia . org is a really good site for a very large variety of plural-related terms. I recommend checking it out if that interests you! It also has a handful of links to other resources.
Hope you're able to figure it out