Ah, this is so cool! Finally, Dragon's backstory is starting to come together! Roughly:
So Dragon sees Roger's death 22 years prior to the start of the story and likely decides to see the world yo understand it better (he obviously would have been exposed to the rough side of it due to proximity to Garp, who hates the seedy side of it). Within the next year or two (based on Robin's age, and Ohara happens when she was eight) witnesses Ohara and decides to form an army. In the next three or four years, he meets Luffy's mama and does *bow-chika-bow-wow* and out comes Luffy, but by this time he's already a wanted criminal so he can't take care of Luffy and leaves him with Garp for Luffy's protection, likely from the WG.
Dragon's mystique is up there with Shanks to me. Can't wait to see it all play out!