I pointed out to Keito that his post made it sound like he was 100 feet from the gates before Kirk made his post. Furthermore, I am 100% sure from just the SL map alone that Iwagakure is not that close to any border. Not even the Stone are so dense as to put their village right at the border of their massive country.
Now, if we're going to get technical, the attack uses malicious sound waves to rattle and shatter the physical portions of the body. This includes liquids, solids, and gasses (though due to the distance between gases they would not be rattled nearly as much, merely shifted somewhat due to the energy passing).
By the technique's theory, the denser the object, the more damage done. An ordinary sound wave has the power to deliver concussive force, and shatter glass, so consider an amplified version of that that can shatter bone in a similar fashion. Sound waves, in theory and in practice, have the potential to put out a fire:
http://www.newscientist.com/blogs/nstv/2012/07/how-to-extinguish-a-fire-by-blasting-sound.htmlIt would take increased chakra in order to sustain a fire under these conditions, so in theory it is survivable for fire-body if he dramatically increased the amount of chakra he puts into the technique. He would then need to reform back to his normal self and counter Kirk's sealing fuinjutsu.
Manji would be approaching Trev, Keito, and the gates area in general from a different direction than the Long Road (since we can consider that the "front" of Iwagakure). Since where exactly Manji is is not determined by the time the attack happens, Manji would have had to be directly between the gates and Trev-Keito in order for the bat attack to affect both Keito and Manji in the same directional sense, even if Keito was only 100 feet from the gates.
As stated earlier then, it is nearly impossible for Keito to be in danger but not Trev (sound armor or no, Kirk's character is not likely to know that right off the bat, and such would factor in damaging him as well if possible).
The above paragraphs are further reinforced by Keito's attack being countered; his attack more or less goes in a straight line, for in order for Manji and Keito both to be affected along with this attack, Manji would have to be almost directly in front of the oncoming technique in order for the attack and for Manji to be affected, and because of Keito jumping like a frog backwards, he would also have to be struck in the same line. Trev, since he has not moved, would then be between all of this madness, and would find himself ALSO having to deal with the repercussions of Keito's attack backfiring, if it were to backfire as stated.
Tl;Dr: Keito, Manji, and the shuriken attack would all have to be lined up, more or less, in order to be hit by the attack as stated in Kirk's post. Since that is unlikely but not impossible, it is very unlikely that it could hit them both and not Trev.