So I sort of understand the story they presented, however I'm unclear on the plans that Xehanort executed over the years. From what I gathered, the reason Ansem SoD sought after Kingdom Hearts and the 7 Princesses of Heart was so that he could recreate the X-blade, but didn't he need the 13 darknesses to do that? This brings me to KH2, where Xemnas wants to create Kingdom Hearts so that he can transfer himself into the Organization XIII members and thus have the 13 darknesses to create the X-blade. Why would Xemnas do that if he no longer has the 7 Princesses of Heart? Also why does he even need to transfer himself into the Organization, when in KH3D he already has vessels scheduled to appear?
So, first and foremost, Xehanort's goal is an apocalypse so he can see the creation of a new world. He had many plans to make that goal a reality and sometimes those plans overlapped.
His first attempt, that we know of, of summoning the true Kingdom Hearts and with it the X-Blade came in Birth by Sleep. That plan failed.
The second, third, and fourth plans ran concurrent.
One was create a false Kingdom Hearts out of the hearts of fallen worlds that could be found behind the doors to darkness. The spread of the heartless and the search for the 7 princesses to open the path took about 10 years, and Xehanort's Heartless was manipulating Maleficent to do the brunt of the work. This was the plot of the first game, and even though Xehanort's Heartless's false Kingdom Hearts was summoned, it was destroyed when Sora sealed the door to darkness and its depths were never fully charted (Many believe the Kingdom Hearts Xehanort attempted to reach in this game was the true one, but the chronicles of 3D confirm that it was a fake.)
The third plan included the construction of the Kingdom Hearts of peoples' hearts, and the 13 nobodies that became Organization XIII. Organization XIII was a failure as many of its members were too untrustworthy or weak to become Xehanort's new vessels, and Ansem the Wise blew up Xemnas's Kingdom Hearts, freeing the captive hearts.
The fourth plan was the most elaborate of all, and involved summoning the true Kingdom Hearts by the union of seven and thirteen. The first trial of this was to see if the princesses could be found, and Xemnas creating the first Organization. When it was clear that there would never be 13 darknesses without further manipulation, Xehanort transcended time to make sure they made up the numbers on the day when the true Organization XIII could be completed. The seven princesses of heart aren't absolutely needed because there will be seven guardians of light to take their place in battle, but they are the backup should the heroes of light choose not to play.
(which they can't afford to take that route, because should the princesses be struck down the entire realm of light will fall to darkness)
And finally, did Master Xehanort transport himself to the past (before BbS) to start scheming from back then? Young Xehanort (is that Xehanort in Terra's body or no?) says that his most distant future self was set to return, and then we see Master Xehanort
As Chaser said, the young Xehanort we see is the teenage version of Xehanort who has yet to leave Destiny Islands to begin Keyblade training, he makes that perfectly clear when he shows Sora the day he met his future self.
As for Xehanort from the most distant future, that would be Master Xehanort apparently. Upon recompleting it seems he's assumed his elderly form, which seems odd but Yen Sid did seem to allude that it would turn out that way.
As for the robed figure, it's the exact same Ansem Seeker of Darkness that confronts Sora in the first game in the secret place. He came to Destiny Islands to set the events into motion. This killed several birds with one stone. He returned to his home world, the Terra side of him saw Riku again, and he confirmed that Kairi had been sent to this world. He waited for the storm to hit and entered the realm of sleep with Sora and Riku, thus allowing Young Xehanort the ability not only to transcend time but to track Sora in the Realm of Sleep.