One-part version: just on the wrong side?

A friend of mine read the one-part version and said: I don't buy it.
Well, I don't buy it either. I wonder if part of my struggle in choosing is that I am unconvinced by this version.
And now, looking back on that scene in Mirror, I wonder if the flaw in the story is that we are witness to Sophie living that other life. Perhaps it would be better if, like Ignat, we are only aware of it from this side?
(This does mean that we are not witness to any less detail about this other life, only that we see from Sophie on this side.)