Osiris was most definitely not responsible for the Aegis, but I do believe he warped into the past during the OG fireteam's Templar battle. This is then when the red-eyed Mind (can't be the Templar, it has blue eyes) we see through in the trailer captures him or takes him out of the Vault. Then again, I guess we won't find out until the DLC comes out. The Exo Stranger could theoretically be anyone, but I assume she's there to get rid of the Heart, and its gone, so the personality we know won't return unless it gets reborn. The Heart of the Black Garden is most likely a worm captured by Quria, Blade Transform during the Vex Incursion on Oryx's Throne. It might have been stored in the Black Garden because it would've been easy to integrate it? Maybe it was stored because the Black Garden isn't like the Vault or Pyramidion? The Garden still exists because the Vex didn't create it. [quote]There are gardeners now. They came into the garden in vessels of bronze and they move through the groves in rivers of thought.[/quote] And, if anybody actually payed attention to the Vex in the Blood in the Garden mission of the NTTE quest, the Vex DID thank us. They didn't SAY it, but they didn't shoot at us. The Descendants literally let us into the Black Garden without opposition. The goal was to anger the Groundskeeper, and we did, and thus the Vex took a collective sigh and continued the fight against us.
From what I understand, the black heart in the black Garden was a worm. The Vex worshiped it because the hive worship the worms and are given power above that of the laws of the universe. The vex wanted that power, so they got a worm, and started worshipping it.