Okay, my theory has changed alot but for now i'd like to state a part of it.
Okay, I have recently figured out that if you give in to darkness, your eyes (for some reason) turn orange. If you want proof, look at Riku's eyes before he gives in to darkness, while he is in "Ansem's" body and after he returns to normal. They go from blue to orange. So, if Terra's eyes went from blue to orange in the FM+ SE then that means he gave in to darkness. Since he is trying to stop the old guy, darkness must have been his last resort just like how Riku could only beat Roxas with the power of darkness. Okay I said i'd only tell you that but w/e.
I have also noticed that When Sora and Roxas battle. Roxas asks "Why did he choose you?" I think the "he" is Terra. But then in FM+ when Sora battles Terra, he says "I did not choose you." Soooo there must have been a mix up or something. Oh, I know. In the first SE Ven takes the keyblade that Riku now uses as with Terra and Sora's keyblade. When "Ansem" got Riku to give in to darkness, he used the keyblade that was suposed to go to Sora. Then with the mix up, Sora still needed a keyblade and since the one that Riku was'nt meant to have was from the road of dawn, it was meant for someone who was filled with darkness so it went to Sora. You see, in the SE, each of the knights came from different roads. Aqua = Light, Terra = Dawn, [Ven]tus = Darkness. Do you get what i'm saying or do I need to explain better?