I totally agree with what you say about humans thinking they're the be all and end all and it was this aspect that initially made the decision very hard for me. I had nodded and agreed with all of Solas' talk of spirits up until that point and so when I was making this choice I tried very hard to figure out if my inclination for making Cole human was based out of this kind of superiority deeming humanity the better option. But, having read Asunder and played through the game too many several times, I do think Varric was correct that humanity was Cole's initial choice.
I see where you're coming from, but my interpretation is different. Cole has changed since Asunder in some huge ways -- he knows what he is now, he starting coming back into his powers, he sustains his physical form through his connection to the Fade instead of having to cater to its needs for food or rest, he no longer fears fading away or not being real, he expresses no distress over most people not seeing or remembering him. Making the big choice based on the past fears of a twisted and broken creature that did not even know the most fundamental fact about his own existence isn't something I'd be comfortable with.
Cole as we meet him during Inquisition knows and accepts what he is: an embodied being from the Fade. He firmly and fearfully rejects the "demon" label to the point of repeatedly calling for his own destruction if he ever slips again, but he is not in denial or distress about being a spirit. I don't remember him ever expressing a desire to be (more) human, which is why that option during his quest kind of felt like it came out of the blue. Varric really is talking out of his arse, IMO, because Cole himself says he came through to help, not to experience mortal existence. When he could not save the original Cole and embodied himself in that poor kid's image in a desperate guilty attempt to make up for his failure, it had disastrous consequences for himself and others. On the other hand, I do remember several instances of Cole calling himself a spirit and wanting to be a spirit (as opposed to a demon).
Some people see the more-spirit path as regression, I call it a hard journey of reclamation and rediscovery. He has to pick up the pieces of his existence, shattered both by his own attempt to live another person's life and by Lambert's revelation, and put himself back together. He chooses to separate "demon" from "spirit", to guard against the former and does his utmost to be the latter even though he is still confused, insecure and hurting, all of which lead to making mistakes. Some of his own hurts start to heal over the course of the game, I think, when he finds acceptance, trust, friendship and many opportunities to help. But the worst and deepest wound is that terrible shackle of loss, failure and hate that binds him to both the former templar whose neglect "killed him" and to his intimate identification with the original Cole. Forgiving the templar and forgiving himself and allowing himself to truly let go of that assumed identity are the two steps necessary to complete his healing, to turn away from the demon he could be, and fully find himself again. They handled the part with the templar perfectly, but dropped the ball on the second part of the equation, the post-Temple of Mythal cutscene, which is such a shame.
I agree that it seems more effort was put into the human path, perhaps this will be the 'canon' choice for future games? I think if the spirit path was more about making Cole immune to possession on his own rather than relying on an amulet I'd be a lot more in favour of it.
I hope it won't be canon, and not only because it isn't the choice I made. I loathe the notion of a singular "canon" in these games in general because it begs the question of whether the choices we get are just red herrings and why we have them to begin with. They have to have a default world state for "late adopters" who didn't play a prior game, but that shouldn't be considered the same as the "right way" to play.
And yes, the emphasis on the amulet feels kind of gimmicky and cheapens Cole's own accomplishments and inner strength, which he does have regardless of where he turns. I wish I could express my appreciation for how far he has come and everything he has done for us, but when I tried to tell him that he helped save the world at the victory party, the useless dialog wheel turned that into essentially dismissing him and his feelings. Thanks, writers. (A perfect example of why I'll never stop wanting the old "proper" dialog system back.)
I think it says a lot about the writing and the quality of Cole's character that he provokes these kinds of discussions 
Definitely!