I already set myself to be disappointed with the triangle by the time the deal with the demon was getting more prominent.
Chariakko is one of those ships that has more than enough basis and it's interesting to explore, but seems to require a certain level of skill when doing it in fanfics. I still appreciate she at least tried.
Anyway, from how the things were going in the last chapter she posted, she seemed to be going for a yet another Diakko end and the explanation they were giving seemed to point out that the demon did it, though even then, it seemed that Chariot's feelings were real and sincere despite everything. At least it explains why Diana was very OOC earlier, though. However, Croix was pretty nicely written from a (pleasantly) platonic PoV.
But yeah, she also made it look that what they did that night came off as wrong and had a certain level of seriousness, like I was expecting Akko, Chariot or both of them to come out clean to Diana about it, but then again, if the demon did it, it sort of cheapens everything.