Hmm...

Not when the one in isolation is the real one or when every Hyung (including the clones) gets that personality trait (like this it would become harder and harder to tell the difference between them).

If none around changes then the logical assumption is that the one in isolation would be the real one. Also, those traits wouldn't be transferred to the copies because they are a specific personality traits. That's the whole point and basis of this line of thought.

"If none around changes then the logical assumption is that the one in isolation would be the real one"
Yes, that's exactly what I wrote ;). It's just that he wouldn't know because he has no contact to the one in isolation and therefor wouldn't know how it works. He wouldn't be able to figure out that the personality traits go back to the original.
The second part I wrote was just an additional theory.

He would be able to because that individual would gain those traits, and if none of them are then it's the isolated one gaining them. It's really not that complex. He could start with the overly scared and aggressive ones first. Too bad no one really knows what personality the first one that died was

I don't think you understand what I mean. If one of them dies and no one gains that trait than how is he supposed to be able to figure out that all of them are just different character traits in the first place? He has no idea at the moment that this is even possible. And even if he had that theory, just because nobody changes doesn't automatically mean that the one in isolation is the real one. The theory could simply be wrong and nobody could've changed.
On the other hand, just because somebody changes and gains that character trait doesn't have to mean that the theory is true. Flowerhead could simply be messing with him.
Besides, having an extreme personality at this point means nothing. Nobody knows which personality trait the real one would have and how another trait would influence the behavior of the real one.