That's part of the thing. Her absorbing the technique is what will allow her to be devoured, as now it'll be inside of her and she'll be dealing with the hatred. The hatred doesn't come from the attack itself, but rather, is a product of the spell absorbing the hatred of so many demons its defeated. So in that sense, her negation would fail terribly. And absorbing 5-A energy doesn't account for absorbing a massive dragon larger than the moon. It's not something that she can just hold her palm out and absorb in a second. The thing will have plenty of time to devour her. If you can't show her absorbing something that massive in an unreactable timeframe, then there's no reason so assume she can here.
Nah, the attack would do some pretty extreme damage. Even without using the fact that a oneshot is at least 7.5 times whatever is being one shot, the way spells work in Gash bell is on a hierarchy (among other things like heart power). Gash's other golden book spells can harm Clear note, and this spell is massively superior to them. Despite them all being Shin class spells. And keep in mind that Gash's durability is higher than his AP due to being able to withstand attacks that should disintegrate his body with just his body alone. That one healing spell healed Gash in his totality. 4x isn't enough to one shot. So Gash will be fine.
I mean, when Kiyo starts the battle and asks himself how to win, he will know immediately what to do. He could cast the body puppetry spell and force her to closer her eyes. Clench her fists. Etc. Having a plausible answer for what Kaguya can do is all that's really needed. And given that the match isn't a stomp, he'll go for the only path to victory because his precog literally tells him that's what it is. Or if his mind manip works since it affects even those who know that its mind manip, then he would go for that.
Also, you've mentioned that Kaguya could start with like three different things now. IT, dimensional warping, shinra tensei. And I could've sworn that in other threads, it was argued that she doesn't start with IT.
Also, the battle takes place on an uninhabitated planet, with no god tree if it's necessary for IT.
All in all though, Thank you for your vote.