There is something interesting about the theory, but I'd say the biggest argument against it is Cole. Including him in your party and having you have such a similar story seems like poor writing, and likely to provoke fans.
I think if the theory
was true, there'd
have to be someone similar in the Inquisitor's innter circle. Otherwise, how would series newcomers recognize that it's a possibility?
If the theory was true, and I knew nothing about the previous games, the big reveal would come, "turns out you were a spirit all along, sharing memories with your host!" And I would genuinely think it was deus ex machina. I would have never seen this happen before, and I probably skipped the codex entries that would explain it happened before.
The only way to avoid all that is to make the necessary codex entries into a person. People are more likely to talk to a person than to read a codex, and they're even more likely to talk to that person if they're a recruitable companion.
Newcomers wouldn't have read Asunder, so they wouldn't be the ones saying "begone, demon serial killer, you killed my beloved Lambert." They'd say, "cool, another companion to recruit, he seems nice. Spirits are the good ones, right?" They'd probably get several chances to talk about his past and what his experience was like, so they could see the parallels.