But there's no evidence for that - it's the equivalent of assuming that all of the spirits are just one person who is really good at dressing up and is using a creative series of mirrors.
There is evidence. The codex for Ash Wraiths and the Holy Brazier state that the ash wraiths in the temple are the spirits of people who committed to serving Andraste. But the spirits of the people in the gauntlet made no such commitment. So it is logical to conclude that these spirits are of the actual historical people (since they have intimate knowledge of them), but if you fail their question then they would not turn into ash wraiths, rather an ash wraith would be summoned by them to protect the Urn.
Well, if Oghren's hypotesis was a wrong one, the guardian could easily tear it apart with a simple no. His answer, 'we do not question the will of the Maker bla bla bla' doesn't sound like a 'nope, you're wrong' but more like a '****, he's on to us! Evasive Measures!'. Not to mention that his 'mind reading mojo' is exactly the one Cole seems to have, So Fade spirit bound to the ashes, which are in turn juiced up by the lyrium present in Mythal's temple. Oh, I mean in Andraste's temple.
And maybe the guardian is only powered up by the lyrium in his body(Like Leli), or the lyrium in the ashes, or maybe he took the ashes and some lyrium when he left for greener pastures.
Actually, no it wouldn't. The Guardian saying no would not disprove Oghren's hypothesis at all. But the Guardian's honest answer of not questioning how it's possible does not confirm Oghren's hypothesis either. And if the Guardian was trying to deceive Oghren, why didn't he tell him he was wrong?
His mind reading is not the same as Cole's. Otherwise he would have known about the dragon's blood. He explained his ability. He can read your past based on the marks they leave on your face and heart. Metaphorical I'm sure, but his reading seems to be focused on possible regrets or insecurities. Cole reads pain and intent.
Cole isn't possessing anyone, he copied the original Cole mind and body when he manifested outside the Fade, somewhat like a Shade. Like how Envy copied the Lord Seeker.
It's revealed in his personal quest with the ex-templar that he is possessing the deceased Cole's body. He didn't remember that he was until seeing the ex-templar because "reasons". He's kind of like Rainer. As Cole died, Compassion became him so that he would live on.
I feel I should say that the text in the operations in questions indicates the guardian it talks about fled a very long time ago, following some attack at the site of the sacred ashes. The papers he thought to preserve have crumbled, indicating it happened a very long time ago. It sounds to me like this was a loyalist that fled or was driven off when Kolgrim's anscestor turned the rest toward dragon worship, not the Guardian we met in Origins.
Or maybe he fled with old and fragile documents that simply did not survive the trip. I suppose the text is too vague to confirm if that was THE Guardian or not, but Varric's narration in the Keep does confirm that the Guardian and Sacred Ashes have disappeared from that location.
Personally, I think it is the Guardian. No one guarding that temple would have fled an attack on it. They would have defended it to the death. And why would the person think there would be others following him there? I think the Guardian fled Corypheus, who may have arrived early to taint the lyrium in the mountain in preparation for the ritual. That's the only kind of attack I could see him running from. If the lyrium in the mountain was being corrupted red with the taint, the Guardian would sense it and move the Ashes so they would not risk corruption.