Retribution isn't even on Roshar anymore, he mentions that he'll have to leave an Avatar in control, that wasn't his exact wording but it's clearly what he meant. So it's a safe bet the Avatar is actually dishing out the Investiture on Roshar atm. From what little we know of Avatars, they're distinct beings. Autonomy had to "instill a powerful dislike" for Hoid in one of their Avatars to deter him from influencing it. From this we can infer that the Avatar on Roshar has independence and choice of its own.
So it's a pretty safe bet, imo, that humans will get access to Warlight soon enough. They just have to convince a piece of Retribution, separate and distinct from Taravangian, to grant it.