So, if Xemnas and Ansem SOD were defeated then why didn't he come back as Terra-Xehanort instead of Old Man Xehanort? And at what point in time were Ansem SOD and Xemnas brought to the present time. Im confuzzled.

I think this is similar to how when Ansem SoD possessed Riku he eventually looked liked Ansem and not Riku, despite Ansem still possessing RIku's body. Both Xehanort and Riku can use the power of darkness to change their bodies appearance.

 

This reborn Xehanort has the hearts of Terra and Eraqus inside him as well, but is dominantly Master Xehanort at the moment. During KH3, it would be cool if the most future version of Xehanort changes his appearance from looking like Master Xehanort to more like Terra, to show the conflict that exists within his heart.

 

So 13 Darknesses could be:1) Reborn Xehanort (formed from the defeat of Xemnas and Ansem)

2) Master Xehanort, from BBS era (only has his own heart)

3) Xemnas

4) Ansem SoD

5) Xigbarnort

6) Young Master Xehanort

7) Saixnort

8) Apprentice Xehanort (I don't think he will risk Terranort since Terra has some control over his body in this form)

9-13) Unknown

 

All of these are distinct characters. The reborn Master Xehanort would be the only character who has conflict between his Terra/Xehanort parts, which sets him apart from BBS Master Xehanort (also BBS MX would be physically frail, whereas the reborn MX would be physically strong). Apprentice Xehanort would look like Terranort, but not have any conflict between his Terra and Xehanort parts, so his personality would be distinct from the reborn MX.

Time Travel, it screws up everyone logic.

Exactly, by logic Terranort should be back since Xemnas and Ansem SoD were his nobody and heartless, NOT Xehanort's. I don't think this was addressed in the game at all, but I want to think that either:

1. When Terranort came back Xehanort and Terra separated, and Terra returned to Lingering Sentiment.

or

2. The Xehanort that appeared was from the time before he took Terra's body (though I don't think this is likely, otherwise he wouldn't know the fates of Terra, Ven and Aqua.)

7 years too late but better late than never.

So when Ansem and Xemnas were destroyed, regular Terra came back as well as Old man Xehanort. What Xehanort did was trap Terras heart in the Guardian leaving Terra's body available as a vessel again. I don't believe Terranort is among the time travelling Xehanorts but the reformed Terra who got possessed again and had his heart put into the Guardian for KH 3.

 

Long story short they both came back when the Heartless and Nobody were destroyed. And Terra was instantly taken over again as this was one of many plans down the road that Xehanort had anticipated.