You are correct. This is why the vote is to use Kawarimi to swap positions with Dummy's stomach acid. (swap into his insides, try to drag out as much acid as you can)
Though it makes me wonder if we can throw stuff while shunshining to boost it to railgun speed.
Uh, Kawamari is
also not a space-time teleportation jutsu. It's just Shunshin (Technically, it's a derivative of it, actually), but a lot shorter range, takes a lot less chakra, is a good bit faster to pull off (to the point where people use it reflexively), and moves the target item in the opposite direction so it lands where you started, however the fuck that's supposed to work. If you're particularly good with it, you can slap a few explosive tags on the log/whatever as you pass it.
You still need to move through the intervening space, and so does the target item. Also, for all that fanfiction has made it out that kawamari with a person is totally a thing you can do, canon doesn't see it used that way at all, much less with someone's insides. That doesn't mean it
can't be used that way, of course, but again, that's up to Chibi. EDIT: NVM on that last point, I'm an idiot and forgot it was already used that way in this story, so Chibi's already okay'd it. On people, at least.
We don't need to try to kill this thing from the inside, guys. We're smarter than it, and we're probably faster than it too. Pick a fight with it, test out our new demony powers, and if things start to go pear-shaped, bail. Just because we start to fight doesn't mean it needs to be to the death here and now.