I've always liked the idea that the Tuskens & Jawas descended from a pre-apocalypse common ancestor.
It's long been my head-canon that they're like Tatooine's version of the Morlocks and Eloi, where one group stayed close to the retreating surface waters until they became too saline saturated and toxic to support life, forcing them to eek a living on the dunes and rocky wastes that their physiology and culture had already become adapted to. While the other group retreated underground, depending on their machines to live, seeking out hidden subterranean aquafers, becoming smaller, more adapted to the tight spaces, low light m and intensely close communal living. Eventually being forced to venture back to the surface to seek out salvage to repair and maintain the ancient machines that keep their vast underground warrens alive.
As for Jawas being seen off world: unlike the Tuskens they're not shy about interacting with the offworlders, many even dwelling in the cities. Presumably this has been true for generations, perhaps even millennia as it was in the old EU. is it really so hard to believe that over that time several clans have managed to buy passage off world to other less harsh backwater planets on a bulk freighter or something?