At this point it is indisputable that Riven killed Yasuo's elder (now named Elder Souma). With that in mind, I am confused about the continuity of events that take place for both of their lores.
In Riven's lore, there was still an ambush/battle, a call for help, and the chemical bombing. This is fine. The problem comes with Yasuo's lore. In his lore, just like his previous bio, he heard of the nearby war-band and left to go fight them. In his new lore, Yasuo comes accross a decimated battlefield and he assumes that something terrible happened (chemical bombing presumably) that only 1 sword could not stop. The next day he returns to the school to find out that Souma is dead and rest of the lore is pretty much the same.
**I want to assume that the battlefield Yasuo ran into was the same as where Riven was betrayed, and if that is the case then when and how could Riven have killed the elder?**
Again this is assuming the battlefield is the same location, and it is entirely possible that it's not. The reason I think this is because Riven's background is the only area that mentions both Noxians and Ionians suffering from the chemical bombing. Following this logic, and with Riven killing the elder, the timeline doesn't add up. For the battlefield to be the same, it would be impossible for Riven to leave, kill the elder, and come back. Also assuming that Yasuo was with the elder, or in the vicinity, before he left, that would mean the elder died before or during the time the battlefield was bombed. The only other thing I can think of is that when Riven flee'd, she found Souma, the two fought for some reason, and he died, but this suggestion just sounds weird.
I would love to have a small timeline that links their lore's together. Again, this is assuming that the battlefield is the same for both lore's. If the battlefield isn't the same, then the continuity is fine.