If Shuuei comes with us to see Yukari, it'll likely come out then. We'll hopefully know more about Koishi by then, too (remember, Iku doesn't know what Koishi's closing of her third eye did); we'll need to stop by Maribel and renko's to get Chen, and Shuuei obviously won't want to accompany us for that (we can just tell Chen to go back to Yukari, then meet back up with Shuuei and take her there). Hopefully, Utsuho'll be there and we can ask her about Koishi.
Oh, and just so it's clear, my intention right now is that Iku's wondering if Koishi might no longer have direct mind-reading ability but still have enough of a remnant of that power to do what Iku's after. Out of character, the idea is to get her to discover what actually happened to Koishi's ability, which would likely work even better.
>"I doubt you'll be seen, but we can take precautions. How about this: I'll go find Chen and send her back to Yukari to deliver the messae that you'll be coming in person. I'll have her tell Yukari to make sure there's nobody else around. Would that suffice?"
Edit: Yes, I think it's best to look for Utsuho before speaking to Yukari. Although I suppose Yukari may very well also know about what happened to Koishi's power.