If the best shape for helping people were that of a druffalo, Cole would've chosen it instead without thinking twice.
It doesn't really matter to him.
In that case, the game would end at Haven because druffalo-Cole would roflstomp Corpyheus and his dragon without breaking a sweat. 
so no matter what choice you make, he remains having some humanity despite how much further you push him toward or away from it.
I'd argue that this is more a matter of being a person than of humanity. (And I really want a "person does not equal human" dialog in the game, grr). But that maybe semantics.
Yep, he does express that he's happy to see you, but I was comparing with a book. He's so incomparably more attached to Rhys there than to anyone in the game. He wanted to protect him all the time, he felt bad if Rhys didn't talk to him for a while. That was so very touching.
Hmm. If he's so broken/needy/clingy/childish in the book, I'd probably read it less as touching and more as weird. I like a more mature and equal bond. There's probably also a lot more space/words dedicated to the relationship between Cole and Rhys in Asunder, than Inquisition allows for the relationship between Cole and the Inquisitor.
Plus, Bioware has this old issue of making PC/NPC bonds feel awfully one-sided because we do all the emotional work and support and showing of curiosity but basically get none of that in return. Which is ... really glaring when the NPC in question is a hyper-empathic, literal embodiment of compassion who fusses over everyone else's hurts but not ours.
So if the bond feels lacking at times, I blame it more on lazy video game writing.
However, I remember that when you turn Cole into a full spirit and come to talk afterwards, he expresses an opinion that your questions are shackles, but still he will reply, because you've been so kind. That rubbed me wrong, almost like a send-off. That doesn't sound like something a friend would say.
Yeah, I did a double-take at that, too, but his happiness and gratitude in one of the following lines and in the Val Royeaux scene dispelled that worry for me. He's kind of right, too -- questions can be shackles (that is what he actually says). Questions that try to pin him down and vivisect him, treating him more like a magical curiosity or a thing to be exploited than a sentient being with his own will. Questions that are more of an "I know you're a demon/monster/parasite" interrogation.
Plus, I kind of find "you kept me in kindness" touching. Cole's quiet voice and poetic alliterations are so bloody soothing.
I'm glad you think so! Now I know I'm not alone. I keep laying that quest aside, because I don't know what to do.
Yes, you're far from the only one! There's been quite a bit of discussion about the problems with Leliana's arc and flip-floppy personality.
Is there any reason for such a difference? Does that brief exchange with Corypheus influence Cole somewhat, changing the final conversation later?
No good reason, in my book, no. If you bring him along, that conversation at the celebration is about himself and what Corypheus tried to do to him. If you don't bring him, it's about the celebration, the mood of the people and his reaction to that. (Regardless of whether he's more-spirit or more-human.) The thing is, the game heavily implies that even the companions who aren't in your party are nonetheless present for the big story events. So they could have had both the "what happened to me" and the "how the people feel" elements in the post-victory talk, for the maximum amount of character insight. It's a bit short as it is -- more time with our friends is always good.
Well.. Cole can be fast. Very fast. And invisible.
He could leap to the man and put a dagger in his heart within a second.
Yeah, but as I said, that wasn't what it apparently would have been like. It would have been a lot more ... gruesome. And a lot more drawn out.
A compassion\murder spirit doesn't even sound possible. Would he be bipolar? And without turning into a crazy demon? Maybe it was removed because it didn't sound logical enough.
Yes, I can't really see him retaining his compassion-side i.e. the core of who and what he is after willfully torturing someone to death. If there was a textbook example of how a spirit can be perverted into a demon by going against or being forced to go against its nature, that would be it.