After playing Trespasser and seeing how ruthless Solas is towards getting what he wants, even with my IQs who are all on the best to terms with Solas, I'm very afraid for Cole.
If Cole is more spirit, then I'm sure that Solas will use Cole, no matter how much that hurts Cole, against anyone who tries to get in the way of what Solas wants.
Then you missed the entire point of both Solas' and Cole's characters.
The devs have said over and over again that while Solas is willing to do terrible things for what he feels is the greater good... he does it for the greater good. Patrick Weekes (the writer of Solas and Cole) has said time and time that Solas cares very deeply about all living things. (Just check out this interview, time mark 50 minutes onward.) If you bring Cole with you in Trespasser (though judging by your post I doubt you did) whether human or spirit Cole constantly comments that Fen'Harel is not evil. "They've got it wrong, he's not that kind of wolf. He wants to help." Solas does what he does out of a desire to help people, he doesn't do it because he selfishly wants to do whatever he wants regardless of how it hurts others. (Although making a world a
Cole himself is an empath who can sense the emotions and intentions of everyone, including Solas. In fact, Word of God said that Cole knew Solas' true identity all along, but didn't say anything. If a female Lavellan romances Solas, right after the breakup Cole almost reveals Solas' plan to her then and there. And again, if you bring him along in Trespasser, he constantly tries to convince the party that Fen'Harel (whom he knows is Solas) is not evil or malicious, he really is trying to help.
Cole instantly senses who Solas is and what he wants to do, but can also senses that he is kind and compassionate and cares about people and does what he does out of a desire to help. If Solas truly had something devious in mind or wanted to use Cole for any reason, Cole would sense it right away and would resist.
In fact, I doubt Solas would want to use Cole since he had a chance but passed it up before he left. If you talk to Cole after the end credits but before Trespasser, Solas will reveal that Cole knew where he was going and tried to reach out as a friend, but Solas erased Cole's memory of where he was going because he didn't want him to get dragged into what he was doing. If Solas had wanted to use Cole as a pawn, that would have been the time.
So, yeah. Demonize Solas if you want, help Cole settle on the human or spirit aspect of his nature if you want, but don't choose based on whether or not you think Solas is going to try to use him, because I doubt he will. If Solas wanted to use Cole, he would have done so by now. If he did want to use Cole for what he knew was an awful reason, Cole would sense it immediately and resist (since he already sensed Solas' identity and plans before). I doubt Cole's in danger of being manipulated by Solas for nefarious ends, so just choose based on which you think would be right for Cole's emotional well-being, not based on whether you think he'll be used by someone who couldn't and wouldn't use him in the future.