If she legitimately loved the girl, she shouldn't have allowed the relationship to start out of basic human decency.
I'm not saying that Celene was intentionally messing with Briala. I don't think she was having a private laugh every time they made love about how she didn't know. But this is a situation where there was a right thing to do and a wrong thing to do and while I can understand how she made the mistake, I'm not about to try and reward her a third time by fixing that relationship.
I honestly think letting Briala have the relationship may have been the kindest thing to do. Briala presumably needed Celenes comfort just as much as Celene needed hers, and giving Briala love and the promise to help the elves was likely the kindest thing Celene could do. I see your point about allowing it to start violating human decency, but similarly I can see how rejecting her, leaving her with a rejection from the one she loved as well as dead parents, and denying her that love, would have also violated human decency, particularly since Celene loved her as well.
I don't think they should get back together, after what Briala did. I honestly think the best thing Celene can do is find a noble male and secure her power, using it to continue the path of progression and culture she started.
As for Briala, I hope that she manages to help the elves, but I also hope that she gets some form of punishment for allowing so many innocent humans in Orlais to die to help her people, be it the guilt or something else. Sacrificing so many humans to save the elves rather than working with Celene to do it diplomatically makes her a hypocrite for being angry at Celene for killing her parents to secure the power she needed to save Orlais.