Zambezi being saved would not be an anachronism because nothing is displaced throughout time. it would be and aberration because the village not being destroyed is and unnatural alteration to history that occurred because 1942 Amaya convinced Esi to be a to be a totem bearer thereby creating an aberration.
After saving Obama from Grodd, the Legends had removed his memory of the event by using Ray's gun, but then Sara is shown discussing with him about what to do with Darhk. What was the point of removing her memory when Obama's further involvement was required. And as Obama has already forgotten everything, why would he interact with Sarah and what would he understand of all the references. He was not a consultant of guide, he was just a teenager, he would have been apprehensive why would anyone come to him for advice or discussion.
When Mallas eventually appears, all the Legends were there with their 6 magic totems, and Darhk was also there, but no one tries to do anything at all, let alone try to attack and kill him, in spite of all their earlier claims and plans. They all were seeing sort of circus show there.
Gideon should be able to synthesize Mallus's voice; making the John Nobel arc unnecessary.