1= Roxas is Sora's nobody
2= yes
3= yes
4= yes
5= Ansem is the heartless of Xehanort, the real Ansem's apprentice
6= Xion is a replica created from Sora's leaked memories so technically yes and also yes about Roxas
7= Ansem's foremost apprentice and the main antagonist of the series
8= DiZ is Ansem the Wise in disguise
9= Vanitas is a being created by Master Xehanort out of Ven's darkness
10= In the end of BBS after the final battle Ven's heart asks Sora if he could join with his heart. Sora allows Ven's heart to join with him. That's why Roxas took the appearance of Ventus and why Sora can use the keyblade
if you have more question i can easily answer them