I have to ask of Akai: Where does Kir's impending doom fit into that plan? Even the slightest suggestion that Akai is still alive would be enough to set Gin off, as we saw during the Red Shirts case. Kir already mentioned before that Bourbon was dangerous, so clearly she wasn't expecting much discretion or sympathy from him if it came to that.
Appearing as Okiya without the glasses or, better yet, just coming up with another concealing agent of any sort (even a scarf would do) seems like a much better choice than appearing as Akai himself. He had to know that something may go down on the train when he got aboard, so not being prepared for that eventuality seems like a pretty big mistake on Akai's part if we take it all at face value (not that we should, necessarily).
Eventually, if the Black Org knew that Akai were still alive, Bourbon would come knocking on doors. If they ever arrived at Okiya (as Bourbon now has), it may get a bit suspicious given his sudden appearance conveniently right after Akai himself "died". Haibara happens to be right next door, so in fact, one could argue that by appearing as Akai instead of taking other measures, he has actually put Haibara in far more danger by keeping Bourbon around (and by living next door in the first place, one could argue). If you look at everything that happened since then, including Amuro now stalking the detective agency and spending a lot of time around Conan, Akai's appearance on the train actually caused a lot of problems that otherwise would have never come to be. Bourbon was already convinced Akai was dead, so Jodie's friend wouldn't have been forced to be involved, Bourbon wouldn't have uncovered so much about Conan or gotten this deeply involved with Mouri and the others, he wouldn't have gotten this far with everything else that's going on (Sera, Jodie, the FBI, etc), wouldn't have uncovered the trick with Rikumichi's body, along with a litany of other issues that directly arose from Akai showing himself on that train.
This isn't even inclusive of Sera being in danger. Them knowing Akai is alive for sure puts her in a terribly perilous position in and of itself. She is then leverage for him to come out of hiding if they so wish to go that route, or they may have just killed her.
If it were me in his place and I had to throw that grenade and had to be seen doing it (necessary for dramatic effect but little else), I would have done so as Okiya and ditched that identity if I felt exposed. At best, Bourbon would think the FBI, CIA, or some other agency had been monitoring the Black Organization's operation and wouldn't have immediately went to "That's Akai Shuuichi". With this course of action, he at the very least gives everyone a chance (including Kir) to not be put into immediate danger. With what did happen in actuality, though, we have everyone in very unnecessary danger. The worst that would come of it is Bourbon being confident and curious enough to find Okiya (who I don't think he'd even seen in any form up to that point, but it's been a long-long time since I've read/watched those old cases) and find the house he was staying in, but with the comparable little information Bourbon had at the time, he definitely was not in any danger of connecting all of those dots at nearly the same speed he has now.
So, in all, we have:
- Appear as Okiya for the grenade toss and Bourbon is mystified as to who this person is but has no solid leads at that moment. Kir stays alive and entirely out of harm's way as there's no proof or reason to connect Okiya Subaru with "Akai is alive" at this point in time. He can ditch this identity if necessary and go a different route; very few people knew Okiya, and it'd be as simple as saying he found a new apartment to let him move away and out of the lives of the very few that had met him before. At best, Amuro somehow uncovers Okiya's name and location, but he has no conclusive means of connecting all the dots as he still has no confirmation that Akai is alive--and in fact has no real reason to investigate this mysterious grenade tosser in the first place.
- Appear as Akai for the grenade toss and confirm to Bourbon, who believed Akai to be completely dead at that time, that Akai was still alive. This reignites Bourbon's interest in the case, when otherwise he was fully ready to give up and report back to the Black Org that, yes, Akai Shuuichi really was dead. Instead, by appearing as Akai, everyone is in danger: Conan, Ran, Haibara, Mouri, Jodie, Camel, James, everyone around them is now going to be in the Black Org's spotlight for the foreseeable future. This, ultimately, turns out to be quite a wild move on Akai's part, as he unnecessarily led a Black Org member right into Conan's life.
I can't help but assume that Gosho has a better explanation than what we've theorized so far as to why Akai did that. The current situation just doesn't seem very logical or, more importantly, smart, and thus not really in accordance with Akai's careful character we'd witnessed up to this point.
