I was thing it did say Xehanorts avenger so couldn't mean that Terra is Xehanort when he loses his memories because did it ever state that Xehanort did know his name when Ansem found him and to answer why when you fight Terra he hates Xehanort is maybe when Xehanort became a heartless Terra was restored just a though though
Okay here is my theory if the main E.S. soldier is Xenanort then this would have had to taken place in the pact meaning Ven could be Sora's father. Think about it he looks like Roxas, Roxas looks like Sora and there is nothing that says nobodies can look like relatives to the original body. And the producer of KH said Ven and Roxas are related somehow so wouldn't that make sense.