That dream scene was not something that really happened. It was not the real Damian, and it was the Slayer who got head-stomped, not Talana. It was a vision Talana was projecting into the Slayer's head, the last act she did before she died.
The reason it led me to believe it was Talana who Damian stomped was the fact that after the dragon falls to the ground, and Damian stomps it, you return to Broken Valley where the
same dragon lays dead on the Shrine and Talana's body is no longer there. So I assumed that Dragon was Talana. Also, if it was not the real Damian, how did he know the Slayer just had a vivid dream when he approaches you lying there on the shrine floor? The way he speaks of the dream leads me to believe it was actually him.
Even if it wasn't real, and that wasn't really Damian inside the dream, you could see how it would lead one to believe that it could have been real enough for Damian to affect reality through it.
It makes sense what you are saying about Talana dying after the dream, and Ygerna taking over once you wake up. I'm just not sure it's definitely the case, considering there is no definitive proof of any one particular moment where the voice the Slayer hears is no longer Talana, and becomes Ygerna.
