1) Why is Riku blind-folded during KH2? Is there symbolize anything?
I always took the blindfold situation to be that he was covering his eyes because they had turned gold. He put on the blindfold because "his eyes couldn't lie" in that, he couldn't pretend that XH wasn't in his heart when he could see he had gold eyes so he put it on to try and fool himself.
Beyond that, though, the blindfold also repressed the darkness inside him.
2) How is Namine created ? Why is she born in Castle Oblivion?
She was created when Kairi's heart left Sora's body. Since Sora became a heartless and casted off his body, Nami was made as a consequence of Kairi's heart releasing at the same time. PoHs normally can't have a Nobodies, but, since Nami was made when Sora also lost his heart, she was made through him. She was born in Castle Oblivion for no reason other than that it's one of the worlds in which Nobodies can be born in.
3) Is the final battle with Xemnas in KH2 takes place inside Kingdom Hearts?
Yes.
4) How did the StarFruit in Destiny Island actually connected people?
As far as we know, it didn't. It was just a myth passed down on that island, and it's meant to be a symbol of love.
5) Where is TWTNW located ? In Darkness or In-Between?
TWTNW is located at the end of the spectrum in the Realm of In-Between, that is, to say, that it is an Inbetween Realm closest to the darkness.
6) What is the symbolization for orange eyes and yellow eyes?
Being close to darkness.
7) Did Ansem SoD had completely destroyed during KH1?
No, he took refuge in Riku's heart, which is why we see Riku struggling with having him in his heart CoM-Days-KH2. When Ansem's machine explodes at the end of KH2 we are made to assume that Ansem SoD had finally been destroyed.
8) How did Xehanort sent Kairi to Destiny Island?
It isn't specified, but, it was mostly likely by using a Gummi Block/Star Shard, similar to how Mickey got around in BBS.
And how did he knew that Kairi will guide him to Keyblade Master?
He believed that the Princesses of Heart had a natural connection to the Keyblade and, believing her to be one because of the light he sensed within her, sent her out to bring him to one.
Is it the magic which given by Aqua?
No. The magic given by Aqua was what kept Kairi save and brought her to DI specifically, but, it isn't what Xehanort felt or how he sent her out.
9) How did Riku get his keyblade, Way To Dawn?
his SoulEater turned into it sometime during KH2. No explanation beyond that has been given yet.
10) How did Xemnas build the Room of Sleep?
Nobody Powers. Or, Disney Magic. One of those.
and how did Xigbar hear Xemnas is talking
By pressing his ear up to the door and eavesdropping, I'd imagine.
11) What is the different between Pureblood Heartless and Emblem Heartless?
Purebloods ARE the heart, Emblems merely CONTAIN a heart.
Purebloods are emblem-less, Emblems have Emblems.
Purebloods are natural, Emblems were manufactured.
12) The mansion in Twilight Town, is it belongs to Ansem The Wise?
No, he merely found it and decided to use it.