The original post can be found here: Kingdom Hearts 0.2 Birth by Sleep Opening Trailer Breakdown. Check the footnotes for further explanation. For example,  means refer to footnotes  at the bottom of the post.
The trailer begins with Simple and Clean -Ray of Hope MIX- by Utada Hikari. Young Sora, Riku and Kairi are playing on the shores of Destiny Islands. This simply refers to the time period that KH0.2 takes place. After this scene, we have a lot of themes and symbolism throughout the trailer. One of which is Dive to the Heart. And Friendship through Wayfinders. The trailer then recaps the tragic events of Ventus, Aqua and Terra in KHBBS.
In the next scene, Terra, Aqua, and Ventus can be seen in a redesigned Land of Departure world similar to the stylings of Kingdom Hearts 3. Castle of Dreams is also included as well. Followed by the allusion to the 13 darknesses.
Continuing, the trailer pits Aqua against her possessed friends similar to the events of Kingdom Hearts Birth By Sleep; this time she is wielding her fallen Master’s weapon, Master’s Defender while Vanitas is wielding the X-blade and Terra has Master Xehanort’s Keyblade. Aqua is quickly overwhelmed until King Mickey emerges from Sora’s Dive to the Heart to lend her a hand. And the two quickly defeat them.
Next, Aqua is momentarily reunited with Ventus and Terra but yet again she is separated from them due to Master Xehanort. Following the aftermath, she again protects her friends without any regards for herself.
This is the most nostalgic and foreboding scene as Ventus and Terra are safe thanks to Aqua’s help but Aqua drifts away, lost. As the feeling of “what becomes of Aqua” sets in, the most emotionally impactful ending occurs. Aqua opens her eyes and they widen and in the reflection of her pupils a hand is reaching out for her. And it’s Sora!
 KH0.2 BBS follows the events of KHBBS. By the time of the trailer, Ansem has already sent Kairi from Radiant Gardens to Destiny Islands and the scene still takes place 10 years prior to KH1
 Throughout the series, there’s a world known as “Dive to the Heart”. Simply, it’s a world within a dream. A recurring theme with Dive to the Heart is that the dreamer gets to the location by being submerged (diving) in water and drifting down and reaching a decorated pedestal. This world is said to resemble the dreamer’s heart. In the trailer, Aqua is diving to a Sora pedestal. Most likely symbolling Aqua is diving toward Sora’s heart. Various characters have been there: Sora, Roxas, Ventus, Vanitas and now apparently Aqua.
 Wayfinders ( star-shaped objects ) are based on the legend of the paopu fruit that bestows and unbreakable connection among friends which always leads them back to one another.
 Terra, Aqua, and Ventus fought Master Xehanort and Vanitas (Xehanort’s apprentice) at the Keyblade Graveyard. Ultimately, Master Xehanort took control over Terra’s body and Vanitas took control over Ventus’s body. Aqua fought both a possessed Ventus and Terra and won, but at a cost that left her trapped in the Realm of Darkness.
Side Note: The spell that covered her friends in orbs in the trailer is the same spell that she used in BBS to protect Terra from the darkness.
 Castle of Dreams is Cinderella’s world. In KHBBS, Aqua visits Castle of Dreams (In the Realm of Light). In KH0.2, Castle of Dreams is the main world in the Realm of Darkness. In/Prior to KH1, Castle of Dreams (in the Realm of Light) is consumed by the Heartless and sent to the Realm of Darkness. Thus, the events of KH1 are concurrent with the events in KH0.2.
 The 13 Seekers of Darkness are incarnations of Master Xehanort representing the True Organization XIII. In the trailer, the chairs represent the darknesses. These individuals will reignite the Keyblade Wars in KH3 when they clash with the 7 Guardians of Light.
Side Note: The Realm of Light is where all Disney and Kingdom Hearts characters live. The Realm of Darkness is a place where only the Heartless live. (Time works differently there also) If you have seen Stranger Things, The Realm of Darkness is similar to the Upside Down.
Side Note: In the trailer, all the people in the chairs are Master Xehanort, representing that they are all Master Xehanort’s incarnations although they are actually different characters. In KH3D, Xigbar explains that the True Organization XIII are all incarnations of Master Xehanort because they share pieces of his heart.
 My interpretation is that Aqua actually fights them in KH0.2, because she is wielding Master’s Defender as she does in the game. It’s likely not actually Terra and Ventus that she is fighting but illusions created from the Realm of Darkness. Prior to KH1, Mickey goes into the Realm of Darkness to look for the Realm of Darkness Keyblade. It is likely that he gets there through Dive to the Heart and encounters Aqua and helps her as seen in the trailer.
Side Note: Mickey is wearing his BBS apprentice attire and wields the Star Seeker Keyblade, so this is likely the case.
 In KH Re:Coded, Mickey writes Sora a letter telling him that there were people who came before him. People who protected the Light. And now these people are suffering and only he can save them. In KH3D, Yen Sid stated these individuals (Aqua, Terra, and Ventus) who are hurting are waiting for their Birth by Sleep (in order words, their salvation). After Aqua selflessly saves her friends in KHBBS at her own expense, and again in this trailer. Knowing that only Sora can save her. The final trailer scene cannot be put into words as to why Kingdom Hearts can move hearts.