There is a few reasons why Terra cannot be Master Xehanort, despite the similar facial structure:
There is no explanation for why his hair would turn white suddenly after only a few years. If his hair turned white that quickly he would have had to be at least in his late-fourty or fifties. Then, if that's not enough to stroke the fire Terra, as far as we seen has no reason to become the Xehanort we've seen. The entire idea of Xehanort being Terra is when Aqua's armor was referred as an "old friend". This would create a great contradiction that would destroy the series because as we were told his memory was destroyed, therefore he would not remember Aqua and if he did he would have no practical reason to create Organization XIII. Edit: Also if he remembered Aqua would he not also confuse Roxas for Ven?
Actually, the main theory going right now is that Terra and MX make up the Xehanort we know.
Terra had plenty of reason to become that Xehanort. The man that Ansem found had lost his memories, and "Xehanort" was the only name he could remember. The Terra that Sora fights in the secret boss battle of Final Mix+ says that he has nothing left but hatred for Xehanort. Perhaps that hatred was so intense that he held on to the name even though he couldn't remember how he knew it.
Anyway, my point is that by losing his memories, Terra is essentially no longer the same person he once was. Look at Sora and Roxas. If you didn't know who they were or that they were both Keyblade masters, and you looked at them solely based on their personalities, would you think they were the same person? No, because Sora is a happy-go-lucky kind of guy who makes friends with everyone he meets, whereas Roxas is often melancholy and withdrawn. One of the reasons why Sora is so different from his Nobody is because Roxas didn't have Sora's memories, he only went by his experiences in Organization XIII.
Same goes for Terra. He can't determine his actions based off of his past if he can't remember it. Actually, according to the first Secret Ansem Reports, Xehanort's loss of memory was one of the primary reasons for wanting to do the tests; he was hoping to regain what he had lost. But the tests backfired and he grew attracted to the darkness.
As for hair color, Terra looked like he was all set to go after MX when he was headed for Kingdom Hearts. The change in hair color might have resulted from the battle or from coming into close contact with KH.
With Aqua's armor, I was thinking that while he might not remember who it belonged to, he felt connected to it, and that intrigued him. So he would often visit the armor in the hopes that it would help him regain his memories. And he may have felt a connection around Roxas, but he couldn't think of what it could be.
My thoughts on this are that Nomura has been known to post information at times that can be misleading. If they are not heartless then this is yet another plot hole in the Kingdom Hearts series. If they are ancestors of the Heartless then they must be some branch of Heartless, it'd be a different story if they were descendants but since they are direct ancestors it doesn't make much sense that they wouldn't be the same race as their descendants after them :bored:.
I'm pretty sure that when Nomura says that the enemies aren't Heartless or Nobodies, he really means that they aren't Heartless or Nobodies. Like when he said the 14th member isn't Namine. He's just flat out telling us because it's not spoilerific and we won't waste our time on such theories.
Darwin said we were descendants of apes. Does that make apes a type of human? No. Even if these creature are direct ancestors of Heartless, that does not make them the same creature. If anything, it makes the Heartless a branch of the new enemy.
To the picture, that is not accurate because that is a "generic" character face compared to the "cutscene" character face that Xehanort's heartless has. An example would be that Sora's eye color is dark blue when generic but in cut scenes they appear light blue.
The shade of Sora's eyes might change, but his eyes are still
blue. Whereas Xehanort and his Heartless' eyes are two completely different colors.