I don't think he looks like Xehanort. But Xehanort has to be the same Xehanort ES was talking about. Also I remember, Diz did not name the lone beaten up body that was sleep in Radiant Garden. Remember the only thing the lone man could remember was his name and that was Xehanort. I reread the Ansem Report 1 and it never mentioned anything about Ansem The Wise naming Xehanort.
Its possible that it was the only thing he could remember, everything else would have been wiped out. Since your name is so simple and it was given to you at a young age, and you knew it for that long, forgetting your name for a long period of time is unlikely.
If the E.S was not looking mad and angry, I don't think he would look like Xehanort. Although Nomura did say that the 3 Knights were brand new characters, having connections to the main cast by Keyblades or possibly other means, he never said anything about the 2 warriors who beat the crap out of the 3 Knights.
Although I don' think The Old Man is Xehanort, but his eyes are yellow and are more closely related to Xehanort's, maybe he was the one trying to control ES at the end. But The Old Man wore cloths similar to Xehanort's heartless, so he obviously has connection Xehanort. I think this guy is either Xehanort's teacher/Master or possibly a relative.
The Dark Knight did not really have much of an impact in the battle, but he got in some good hits, maybe he is Xehanort and is being taught or being instructed by The Old Man. If the Dark Knight was young, he could fit Xehanort's place. Remember Xehanort was not that old when Ansem the Wise found him. So the Dark Knight could be a young man age range from 17-22.
This could also mean that Ienzo was even younger when he was studying under Ansem the Wise. I'd say around age 14. Then over the years, Ienzo grew up a bit more before his nobody came into existance.
Looking at ES's face, he looks like he is fully grown, age range from 23+, Xehanort was a young man we discoverd. We did not see The Dark Knight's face, so he could of had a young face. Also if this secret movie had something to do with Xehanort's memories, it would fit perfectly that the Dark Knight was the one who had these memories in the first place just, being fully realised.
ES: I see. Your strength is... What I sensed in you is...
(I think Sora's strength is his heart. What ES sensed in Sora was the connection between Sora and other Keyblade wielders.) The Proof of Connection probably demostrates that Sora has a connection to ES and or 1 more person in the Secret Ending.
ES: So you have returned, young Keyblade user. I have no further power to give you. All I have is my hatred towards Xehanort. Will you allow me to engrave that into your heart?
(Considering that the Dark Knight could be Xehanort, he and his fellow soliders probably clashed with him in the past. It seems as though the battle between the 3 Knights and the 2 soliders were going on for a long time. Its possible that they clashed before, after and is going to again.
It's possible that Xehanort did something that was completely out of line and it led to him losing his memories.