I think the message from the Wicked Hearts was that everyone was conspiring like mad. Celene and Gaspard were thinking of ways to get the eluvians away from Briala, Celene was plotting against Gaspard and Briala, Gaspard was plotting against Celene and Briala, and Briala was sneaking around trying to get the dirt on them both. To begin with I felt bad about letting Celene die as it did seem like I was failing in some way but really it was all part of the game.
I sacrificed Celene in much the same way as she had sacrificed her servants to become Empress and the elves of Halamshiral as a show of strength to the nobles. As an elf I felt I was just getting payback since I did know about the elves of Halamshiral. Also out of my 3 advisors and Cassandra, only Josephine seemed in favour of keeping Celene on the throne. Cass and Cullen both wanted Gaspard and Lelianna suggested the arrangement whereby Briala actually got control.
The ending with Celene and Briala seems to put things back to pretty much what Celene was offering Briala in the book. In other words she gives her a title and probably continues with cautious reforms but Briala didn't seem to think it would last before and the closing line suggests as much. The moment Celene got a whiff of revolt by the nobles she would backtrack on doing any more.
The ending with Gaspard and Briala has more radical reform in a quicker time scale with Briala knowing that she has the backing of the Inquisition, which is why we are told the nobles are holding fire. Actually if anyone cared to ask, I would tell them I am doing them all a favour because by improving the lot of the elves it is undermining the inroads that the Qun have made into the lower classes of society. Clearly no one has paid enough attention to Varric's story of Hawke or they would be aware of the danger. Society depends on having elven servants so they can't simply kill them all. If they can see their situation improving under the current ruler, they will have a vested interest in supporting them against the qunari.
That said, the Gaspard/Briala combo only really makes sense and works the best if you are an elf. Otherwise going for Gaspard alone or Celene/Briala is probably best. Celene likes to think she has stayed in power for so long because of her mastery of the Game but if the book is anything to go by then it is Briala's work as spymaster that is the true reason. Hence everything falling apart when she doesn't have Briala around.