So close to what Deva's basically working on, except she doesn't necessarily have a good understanding of the personality of people so it'd be even worse.
I can’t speak for Kako, but Deva doesn’t seem to be the kind to care much for acting, and more of ‘using the forms of others to lower their guard, and once they do, immediately attack them’. Also, she’s FAR from helpless when found out, VERY far from it and super dangerous.
Also, there’s the whole thing of wanting to turn everyone into clones of her, YES
I wonder how, in such a hypothetical incident, Eri would get around ones that can see through deception? (Emma, Satori, etc) Take them out first?
Two possibilities on top of my head:
Gensokyo trusts Emma, Satori, Eiki, etc to root Eri out. Mind readers like them or people who can read energy like Meiling and Lust are Eri’s hard counter. So...
Eri impersonates one of them to take away suspicion from himself. Worst case scenario, Eri can also possess the bodies of one of them, so he not only gets their mind reading/etc, he also takes them out of the picture since he’s inhabiting them. Fortunately, because his possession isn’t mastered, he has to hide his own body somewhere because it would be completely vulnerable otherwise.
He avoids them entirely. Unless they are actively searching for him, they most likely won’t be able to find him in a crowd, and that’s when he tries to incapacitate them.
So yeah, still hard to find him, but they’re pretty much the key incident solvers to beating him.