There's still some unknowns left in the story, as well as an underlying conspiracy.
Sure, Riven taking her sword to Elder Souma to break ended up with him exploding it and killing himself in the process clears Yasuo of the charges against him, since he didn't kill the elder - but it doesn't answer a few really glaring questions.
Why was there a cover up? ? The priests at the temple would have pieced together the story on seeing the blade Asa Konte brought, and seeing the metal shard that matched it that was pulled from Souma's neck and known it was this Noxian blade, and not Yasuo, who did the crime, yet they remained silent, even during Riven's trial.
We know that the bone scrubber who found the missing blade piece lodged in Souma's neck was distinctly told NOT to report it to the magistrates. There was a cover up involved in Elder Souma's death, and it was done to frame Yasuo.
So therein lies the big questions. Why? Why frame Yasuo? To what end does framing the pupil of Elder Souma achieve? Why send Yasuo's own brother against him, and pit them against each other in a duel to the death? Who was the real target there? Who are the priests at the temple working for, that they would orchestrate a framing and a crime like this?
There's quite a few unanswered questions here, and even though Yasuo is innocent of the crime now, someone in Ionia has been plotting against him. Who is it, and why?
Edit: Some minor changes due to me forgetting that the blade was taken to the temple by Asa Konte 4 days before the trial.