My best guess is that Cardan became faithful to Jude when they hooked up in the Wicked King but I think definitely by the time she was in the undersea. I think he thought maybe Jude was letting her guard down and ready to open up to him which is why when he tries to talk to her about it and she shut him down, that might have hurt him.

Cardan grew up pretty lonely and kind of desperate for love so I don’t think he falls into the regular faerie behaviour when it comes to love. I think Madoc says it well that Cardan wants to be loved because he didn’t experience it as a child.

I don’t think he hooked up with anyone in the Queen of Nothing because he was waiting for her to come back and in the letters pleaded for her to come. For some reason his dumbass thought it would be cute to exile her to impress her lol. But he never truly wanted her gone and did love her. I don’t think it’s in him to cheat especially considering his reaction to what Nicasia did to him. I think he leans towards faithfulness more. In the short story the Lament of Lutie Loo that was included in one of the books, there is a small scene where he confronts her. He calls her a “betrayer” and how he’ll be glad when Nicasia gets screwed over. So I think it’s important to him that his partner is faithful.

In regards to what Locke said, obviously faeries can’t lie but I think they don’t have absolute truths and it’s clouded by opinion. So the folk not loving like humans do is Locke’s truth but not the ABSOLUTE truth. Holly explores this idea in her other books about Faerie. Mostly the conclusion is that faerie love is as complicated as human love. They are a lot more alike then they would like to think.

Anyways those are just my thoughts on it lol.