Finally would the Yamasenken help as you theoretically say that opening shout is at least based on kiai sense its supposed to be a paralyzing scream due to surprise.
As far as i read it, it's just supposed to be something distracting rather than kiaijutsu. Ie. a psychological attack rather than physical.
And kiaijutsu isn't just based on surprise, that's basically the amateur level.
Above that is when you get proficient in projecting your voice, at that point you're basically mimicking the sound part of a flashbang, which is enough to
physically stun people for a moment.
Then at the top of the skill, there's the point where you learn how to project the shout as a shockwave of noise. At that point, the stun effect can be serious, able to knock people out, cause minor impact damage and seriously harm or even kill small animals.
(caution, kiaijutsu is NOT safe to practice, even a slight mistake can injure your throat quite badly)