Sora has Ven's heart of light because Ven killed Vanitas (his heart of darkness)
Okay, it started with this:
Ventus was an apprentice to Master Xehanort. When MX was researching the Keyblade War, he stumbled upon a weapon called the Xblade. In order for the Xblade to be summoned, there was to be one heart of pure darkness and one heart of pure light encountering each other, in this case, Ven and Vanitas. When MX knew of this, he extracted Ven's darkness out of him and created Vanitas. Afterward, Ven had a battle with Van, he regained his body, but, after destroying the darkness of his heart, he fell in a comatose state because his heart is fragile without darkness. In this state, he formed a pact with Sora's also incomplete heart and both formed a complete heart.
Summary: Yes, Sora has Ven's heart because he killed Van.
Xehanort has Terra's body
Okay... Masters have the ability of transferring their hearts to another body. Master Xehanort stabbed himself with his keyblade and, with his body's last strength, sent it towards Terra's body.
Summary: as Nomura said in an interview: Xehanort is MX's heart commandeering Terra's body.
Terra's soul is in the RoD & Lingering Sentiment
In Kingdom Hearts, the soul acts as a battery to the body, nothing more. Let's saaay... That Sora has a body and a heart, without a soul, he'd be a vegetable, as in, he can't do anything except think. With this in mind, Terra's soul IS NOT in the Lingering Sentiment nor in the Realm of Darkness, it's still in his body with Master Xehanort in control of it. And the LS is Terra's armor with his strong emotions in control of it, mainly his emotions towards his friends and the departed Master Eraqus.
Summary: Terra's soul is not in the RoD nor in his armor.
Aqua is in the RoD looking for Terra
Aqua entered the Realm of Darkness with her armor to get Terra out from it, but to do so, she had to send her armor with Terra.
Summary: Aqua's in the RoD, but, she's not looking for Terra.
Fun fact: since the armor was in possession of Xehanort, he decided to put it in the Chamber of Repose, possibly to try and remember something.
Xion is Sora's replica & Roxas's Oblivion Keyblade
Xion is Sora's sampled memories put inside a body, so, yes. Personally, she isn't the Oblivion, but rather, since she could wield a keyblade and merged with another being that could, Roxas could dual-wield because of her, not because she's a keychain.
Summary: Xion is Sora's replica.
& of course, Roxas is Sora's Nobody
Yes.
Summary: there's no need for one! xD
And I don't get Xehanort's story after taking Terra's body.
Master Xehanort's and Terra's memories are locked away in their hearts because Terra took control of his body for a small moment and stabbed both their hearts to lock away their memories, so Xehanort is an amnesiac being. Since he suffers from memory loss and he knows it, the logical thing to do would be research about it and try different methods. Since this game was thought of years after Kingdom Hearts/Final Mix, it's logical to conclude that Ansem Seeker of Darkness/Xehanort's Heartless had no interest in remembrance but only taking the "darkness" that was KH. But Xemnas was interested in both remembering his past and creating a world reflecting himself.