Interesting hypothesis but I don't buy it. Keep in mind that Ryo only started having dreams of Shenhua after he discovered the Phoenix Mirror. So how do we know that it wasn't the mirror causing said dreams instead of Shenhua's powers? And if it was the mirror, then how does that relate to the Yanglang scene?
I do think that the mirror is causing the dreams, but why is it only Shenhua who can appear and communicate with Ryo in his dreams as a result of him possessing the mirror? I think that Shenhua's powers and the mirrors' powers are connected somehow and work in tandem to allow Shenhua to appear in his dreams. The two are not necessarily mutually exclusive.
Another question I have to ask is what would it take for people to see this interrogation scene as comedy? For me, this is simply Occam's Razor in action. One way humor works are by subverting expectations. Most of the audience who doesn't have a huge understanding of the Shenmue lore is simply going to assume that an innocent-looking person like Shenhua could never hurt someone like Yanglang.
It does subvert the player's expectations and I believe that some comedy was intended to be derived from that. But again, I don't think that a slightly comedic subversion of expectations and Shenhua using her powers are two mutually exclusive concepts. If anything, I think any comedy that was there was intended to lighten the tone because the scene could have really taken a dark turn if it had focused more on what Shenhua actually did to him.
And if the implication wasn't that she used her powers, then what exactly was she meant to have done to him? Beat him up? Nagged him about shoes and cute boys until he couldn't take it anymore? Shenmue III's writing certainly has its shortcomings, and I can even point to this scene for some examples of that. But I refuse to believe that Shenmue's story has degenerated into completely inexplicable nonsense just for the sake of comedically invoking some kind of silly anime trope. In that case, they may as well have gone all out and had Ren get a nosebleed at the sight of Niao Sun's tits.
What I'm trying to say is that if Shenhua was the only one capable of successfully interrogating Yanlang, then there has to be an actual reason for that regardless of whether it was intended to be funny or not. Since we know for a fact that Shenhua possesses some mystical or spiritual abilities that normal people do not, Occam's Razor suggests that she used one of those abilities to get him to talk. It's the only thing that makes sense.
I also thought that the way the camera zoomed in on Shenhua's eye with the ominous sound effects and lighting, immediately followed by Yanlang's screams, made it evident that she was doing something serious to him.
This also brings up another issue I had with the overall story. If Shenhua was meant to be Ryo's equal, then why is she reduced down to the proverbial damsel in distress? If she could intimidate a strong man like Yanglang, Ren, the Chiyoumen guards or Longsun Zhao himself in the Project Berkeley trailer, then why was Niao Sun able to kidnap her in the first place? She could have easily used her powers to defend herself.
If the answer is that she can not control her powers, then I would dispute that to some degree. In Shenmue II, she clearly states that she used to play with dandelions and provides a demonstration of her powers. This means on some level, she is in partial control of her power. So again, why didn't she use her power to defend herself from Niao Sun?
I've been thinking about this and it's possible that because she does not yet have full control of her powers, she doesn't know how to use them to defend herself against oncoming attackers. This could explain why she didn't use them on Niao Sun. Recall that Yanlang was fully restrained. She obviously didn't cause an explosion or anything, so she probably just did something to him mentally.
The scene from the Project Berkley trailer probably won't occur until near the end of the series. After all, there wouldn't be much point to all of Ryo's martial arts training if Shenhua could simply blow up all of his opponents with a huge fireball. There will probably be a moment during which she spontaneously uses her power to ward off attackers and then she will finally understand the full extent of her abilities and learn how to use them to their full potential.
It's also possible that she cannot utilize her powers in this way unless the mirrors are in a particular location or used/arranged in a certain way.