I’ve just come up with a theory about Kuina’s death. What if she actually died falling the stairs but it was the doing of the sword?
We know that Kozaburo created swords with the intention to kill as many people as possible.
Then I find it weird that Wado Ichimonji has not shown any special quirk so far considering it in the same grade as Enma. In the night Kuina died, she used the sword to fight Zoro and then tried “find a stone to sharpen the sword”.
Could it be that she overdid something or the sword tested her and she failed? Maybe Enma and Wado Ichimonji both like to kill but the former tests any of its new wielder (show-off personality) and the latter seldom does the test but the test would be very hard (quiet personality).