It's an open-and-shut case of not guilty on the grounds of self-defense.
There is no open-and-shut case as the case is not tried in a court of law. It's decided largely by Judge Aunt Eller. Frontier justice as a way of tying up loose ends is a trope that runs through the musical particularly the shotgun wedding of Ado Annie.
The original Lynn Riggs play leaves the question of Curly's possible repercussions with the law unanswered. Hammerstein added the community justice scene, you don't have to call it a kangeroo court if you don't want to.
unlike the musical, the end of Green Grow The Lilacs is left rather undecided as to Curly's trial for accidentally killing farmhand Jeeter (renamed Jud Fry in the musical).
I guess we'll have to agree to disagree. I found Fish's addition to rise quite organically from the musical itself though it was indeed an addition.