I feel like these are exceptional circumstances and enormous changes are a possibility at this time. I think people focus too much on how fast political change has been pushed in the real world, and even then underestimate.
I'm not sure I'm understanding this. Comparing Thedas to the real world is inaccurate, and we can't compare fast political change in the real world to...Leliana's changes happening quickly? I would argue that real world political change is not fast, at least not authentic change. In the U.S., the Civil Rights Movement was in the '60's and we had desegregation, decriminalizing of interracial marriage, increased rights for all citizens. But anyone looking at modern America would not see these technical changes being accepted by everyone. Much of the political dialogue right now reflects the long held resentment certain demographics have had against other demographics. Just because it's no longer publicly accepted to be overtly racist doesn't mean there are no racists anymore.
Thedas has gone through extreme upheaval in the past decade. A blight that could have threatened to wipe out the world was headed off in extreme record time by a small group that Leliana was part of, even potentially lead by her future spouse. When the veil tore open, again Leliana was there and with a figure the people see as divinely gifted. These sorts of apocalyptic events aren't something the real world has to deal with.
We have in game evidence that people aren't going to straight up accept radical change because of the events of the game. If we help Brother Burkel establish the Chantry in Orzammar, he ends up being killed by a mob and there's threats of an Exalted March on Orzammar. This was because his Chantry turned out to be popular and started to threaten large amounts of dwarfs converting to Andrasteanism.
We also have the Inquisitor, the person who closed the Breach twice, walked out of the Fade alive twice, and killed one of the original darkspawn, being held in a kangaroo court in Trespasser out of fear of their power. If Ferelden/Teagan were truly afraid of the resulting uproar from the masses if he shuts down the Inquisition, I think he would have acted a bit differently. But it's two years after the Breach; people are concerned about picking up their lives and moving on, not starting new cults of the Inquisitor. They would rather get back to the status quo than use the recent events as an excuse to reform society.
Now, this could have been implemented well. If the Freemen hadn't been depicted as cartoon misguided villains following red templars and instead been allowed to have actual valid grievances against Orlais' nobility and social structure, we could have ended that quest line by conscripting them against Corypheus and provided limited rights once the threat had gone. A Divine Leliana would work to continue these rights while a Divine Vivienne might have tried to suppress them or go back on the agreement.
But instead, we see no lasting changes in society apart from mages and templars.
The real world is also much more science and logic driven, less likely as a vast majority to see things like that as divine. Under the right circumstances, Leliana has been a part of and is visibly supported by the greatest heroes of the time and was at their side during their greatest triumphs. I think it would be much more difficult for the people to turn on someone like that than someone like Vivienne.
Yeah but human nature is human nature. As I pointed out above, no matter how "logical" modern society is (debatable) individuals are still flesh sacks filled with irrational emotions. It's no longer PC to say racial slurs. But racial profiling is still a thing.
If we're comparing Thedas to "less logical" times in the real world, the masses could just as easily claim that she's a demon in human form (and Trespasser epilogues indicate that this isn't too far from the truth in some world states) or a blood mage, or any number of bad things. How else could a mortal woman defeat a Blight within one year, rise up to be the Left Hand of the Divine, and then soon after the Divine's death take over that role? And magic is a real thing in Thedas, so the argument that Leliana used blood magic to become Divine is no less realistic than to say she's chosen by Andraste. More realistic, IMO, given that the Herald being sent by Andraste is heretical to the Chantry and is meant to be a unique occurance.
One benefit of Leliana's changes is that they're so broad spectrum that there's almost something for everyone. Massive new populations are openly welcomed, allowing Leliana to reach out to large populations for support that wouldn't have otherwise been available. Even human men benefit from this, which would likely give pause for the human population rebelling against her, at least to some extent.
Again, not really how humans work. Just because a law might benefit me, that doesn't mean I don't mind if it also benefits a demographic that I'm against. People aren't rational, they don't want people they don't like to get good things, even if they also benefit. There's a common phrase for it, cutting off your nose to spite your face.
And just because you or I might consider one of Leliana's changes to be beneficial to everyone doesn't mean that people of Thedas will agree with that. The nobility might be just peachy with how they're treated by the Chantry and see no need for changes. Any change for them means possibly giving something up so that someone else can gain something. Why would those on top (and in power) want to change their situation at all?
I think the circumstances of Leliana's history, the balance of her reforms and her overall more balanced approach than Vivienne's in dealing with people will make her a better candidate. I know Vivienne was successful at working the Orlais nobles to some extent, but honestly I didn't see much evidence that she was this great politician. The circle fell apart while she was there, let's not forget, and she doesn't seem at all prepared to deal with anyone she would see as below her station except but to mock them. I just don't see that she has enough experience or flexibility for something like this.
I'm not arguing that Vivienne is a better candidate, I'm arguing that I think she would be a more effective candidate in the long run. More of her doctrines would last past her death, whereas I predict that a lot of the changes Leliana makes would collapse soon after her death, or the death of her successor if she has a hand in finding someone who shares her views.
Plus, how is the Circle system collapsing any indication of Vivienne's political prowess? She has nothing to do with Fiona's rebellion and is not able to control the opinions of every Circle mage in Thedas. If she had been able to single handedly stop the mage rebellion she wouldn't have been a good politician, she would have been a god. The fact that she still has control over the loyalist mages some templars is a better indication of her political abilities.