I'm team Fuuko so if this ends up being a wake up call to Yuni and she goes through an arc of finally realizing she loves Fuuko and not her current GF, I'd be satisfied. The most obvious path this could go down is this new girl serving as an obstacle that makes her realize this, along with more Fuuko backstory.
Same, I want Fuuko and Yuni to fix each other and become a healthy couple.
But forget all of that, what the hell did they mean with that Crown of Thorns imagery? I'm a sucker for religious symbolism and this could mean a dozen different things, especially because it's being PUT on her. I know this will sound goofy but...is Fuuko meant to represent Jesus in this story? No I'm being completely serious.
I doubt the average Japanese author has in-depth knowledge of Christian imagery, and if they did it's weird that they would start using them only halfway through the story. I think they did it "because it looks cool".