Ah, I hadn't seem that tweet clarifying Cole's character and mentality, I was going off what David Gaider said a while back. Yeah, it does actually probably describe Cole's behaviour and character a lot better to say he's simply childlike, rather than child-minded.
I suppose my main confusion was the argument people were making that his actions in Asunder meant he had to be as intelligent as an adult, which is kind of a disservice to how ridiculously savvy, stubborn and determined some kids, teenagers and young adults can be (especially in fiction). 
I don't know about whether or not Cole truly has a soul, since Fade Spirits are supposed to lack them, (unless that's simply the Chantry's party-line and not a definitive fact), but he does seem to have "captured" (for the lack of a better word) the essence of the real Cole and feels the same things that he did. It would certainly tie in with how we saw Justice experience the same sort of thing from the remnants of Kristoff's memories in Awakening.
But yeah, a complicated thing like romance, the nuances of relationships and all the trials and tribulations that come with it, would probably throw Cole through a loop and end up being a colossal disaster for everyone involved.
"To be honest, though, I didn't see much childish about Cole. His social anxiety was not really that child like. Most children don't have the self-awareness to worry that they're going to bug the adults with too many dumb questions. Most don't feel reluctant to insert themselves in conversations, especially if they're feral kids that weren't taught not to. Besides, I can't imagine a child having the level of self-sacrificial idealism that Cole has. A child that saw Rhys as a father, or older brother, would hide behind him in intense situations. They would expect protection. Cole is always the first to throw himself in danger to protect Rhys.
Cole clearly has very little knowledge of how the world works and how to operate in social situations, but that's a lot different from actually being a child."- Nocte ad Mortem said this in Cole's thread, which I feel pretty much the same.
I assume he preserved that childlike tone by social isolation, but his mentality still grows by years of observation and what he saw made him apathetic to life and death, I'am not gonna prettify this part no matter how I understand where his obsession coming from, I'am just glad that he changed.
About what chantry said before, it bugged me off too. But chantry also said that sprits are merely empirical beings, they can't dream, they can't be tricked by themselves, and they can't create the things that they can't even imagine of. But Cole has wide rang of emotions and sensations, he aged, he can dream, he can be reasoned with, he changed his motives and cognitions, which are… pretty much contradict to every part of the chantry's statement. So to me, Cole is definitely unique and intriguing.
Personally, I believe he is Legion in DA, like:
“Does this unit have a soul?”
“Legion, the answer to your question is YES.”
You feel me? XD