1. What did Xehanort mean he had different paths to go down?
Xehanort is referencing the many plans he has made to try and obtain access to Kingdom Hearts, for example creating a Kingdom Hearts through destroying worlds (KH1) or gathering the hearts of man (KH2) or making an army of himself to clash 7 hearts aligned with the light vs 13 hearts aligned with darkness...
Also how come he and Terra do not have amnesia but Terranort does?
This is another mystery of the series. It is hinted that Apprentice Xehanort may not have actually ever had amnesia but it hasn't really been clearly explained by the games. We only have Braig's comments in BBS and KH3D and Nomura's interview comments to go from.
And of course we cannot trust Xehanort's Reports in KH1 since he was proven to be lying.
2. What does all the poetry starting with "Images of their backs, preserved in memory" mean? Like the different lines.
"Hidden truths" - Xehanort has some secrets and plans, Terra has Eraqus in his heart.
"Image of their backs, preserved in memory." - This is a pivotal moment that Ansem the Wise and Ienzo were witness to.
"Two who were never meant to meet" - If Xehanort had never meddled with anything, neither Aqua or Ansem the Wise would have been in the Realm of Darkness.
"All the pieces lie where they fell" - Everybody who is in need of saving...still needs saving. (Throwback turn of phrase to KH2's opening)
"Where they wait for him..." - Everybody is waiting for Sora to save them...give them their "Birth By Sleep"...mend their hurt...do the Jesus-y thing.
3. Aqua is in the RoD after Sora defeats Xemnas and that's when she meets ATW?
Yes. Aqua has been in the Realm of Darkness for almost 12 years.
4. How were Terra and Ven both at Land of Departure and both said Sora? Did this not actually happen?
It's a metaphorical "we are waiting to be saved" thing. They aren't really there.
5. Who did Sora have to go to? And why?
Mickey's letter tells him there are people who are in a lot of pain (Aqua, Terra, Ven, Roxas, Xion, Naminé) and he needs to go to Yen Sid, which he does in KH3D.
Sora has to undergo training to become a Master/gain the power to restore sleeping hearts.