You are missing my point. She would be heartless if she let her parents's murderer walk free without being punished. Briala would be heartless if she allow her parents' death unavenged. Why would she even use Celene as a tool to serve the elves after what she's done in Halamshiral. Celene already show perfect example of killing the elves when the nobles bark her about it, and she's killing the elves to prove her point to the Orlesian nobility. Why would Briala think Celene is trustworthy at this point since Celene choose herself before the elves. When Briala decide that Celene can no longer be trusted with the fate of the elves that mean she's moving away with her own plan or to seek someone else.
Would it? That's a pretty subjective point about what's heartless. That was twenty years ago. Celene's been her lover for just as long and has done more for the elves than any other ruler in centuries in that time. This isn't clear cut for her emotionally.
I've never understood this logic honestly. Why is it either or? If Celene allowed her support to crumble away and be deposed, everything she's done for the elves would evaporate and the elves would lose the only ruler friendly to their plight that they've had in a thousand years. Briala understood that. She wasn't happy about it, but she understood that Celene didn't have much of a choice. She forgave Celene within the day.