As I keep saying. Without Duke Bastien her position as court enchanter means nothing. Without his wealth and influence behind her words the title: Court Enchanter, simply means a mage who is a glorified jester. One who entertains the court. Should she manage to retain the position it would not translate into actual power. Especially seeing as Celene evidently does not find her interesting at all.
So your entire counter argument is, "Vivienne's influence and power is tied 100% to Bastien."? That's a bit of a pity. It would be like saying Leliana's prowess and influence in the Game was tied strictly to Divine Justinia, and once Justinia died Leliana somehow instantly became nothing in Orlais. Yes, they were put into high position by their patrons, but what they made out and got out of that position was entirely on them.
The Court Enchanter was basically a court jester when Vivienne got the job, but she, not Bastien, was able to turn it more than that. She was able to turn it into an advisory position to the Empress and the rest of the Court, and she was able to start gathering political power for herself even though that was strictly prohibited. She started making "friends" with the nobles at court, gathering gossip and secrets to use against her enemies and anyone else that might cross her in the future, and trading favors to get what she wants out of people.
On top of all of that, she was almost able to become the First Enchanter of Montsimmard, but since rebellion put a stop to that, she became the de-facto leader of the Loyalist mages instead. Those appointments had nothing to do with Bastien's influence at all, since the First Enchanter position is controlled by either the outgoing FE or the KC with the approval of the mages, and her becoming the de-facto leader is because the mages are putting their trust in her. That put Vivienne in a big position of power since she has a good amount of mages under her command as well as all the resources of the remaining Loyalist Circles as well.
Morrigan was able to gain power because she could indulge Celene's interest in the occult. Vivienne because as long as she was Bastien's official mistress her words had weight behind them. Vivienne without Bastien is simply a rude conservative mage. Those the court can find anywhere.
How about a rude conservative mage that possibly has scandalous information on you that could ruin your reputation? Or how about said information about a political rival and was willing to negotiate to arrange an "incident" that would benefit you greatly? How about a mage that has a large amount of magical resources at her disposal by being the leader of the remaining Circles that could help you with a certain problem of yours?
The Court can't find those anywhere, and Bastien's death isn't going to make Vivienne less of a threat or possible ally to people. She still has information on the nobles at court, knowledge on how to possibly use it, and the resources to back it up.
Without Bastien she holds no influence or power at all. Anyone who respected her did so because they feared the Duke, and anyone who fears her would happily tear into her now that her power is mostly gone.
What did they have to fear out of the Duke in the end? He was bedridden and comatose at the end of his life, and people still feared and respected her. Were they afraid he was going to magically pop up out of bed long enough to draft a contract with the House of Repose? Summon an army of bards? People who feared her would want to tear into her anyway, and if she put herself in a bad enough position for that to happen, there would have been nothing she or Bastien would have been able to do, even if he was alive and healthy, until it was too late and the damage was already done.
Though, I'm sure they can certainly try to tear her down after he's dead, but I'm not sure it would do much good. Bastien gave her additional resources, but Vivienne was the one that actually had the knowledge on everyone. Just because Bastien's dead wouldn't make any blackmail able material any less true, and good luck sending assassins after her. She's a powerful Knight Enchanter that can freeze a person with a wave of her hand, and she can shatter them just as easily.
She was allowed to arrange his funeral, yes. But that does not mean she gains any power from doing so. All of Bastien's lands will pass to his heirs, which do not include her (since she is a mage), and as previously mentioned, friendship means nothing to the nobles of Orlais if you do not have power and wealth behind you. She can only gain power with the aid of the Inquisition.
Gains no power in doing so? She may not be gaining any lands or title from his death, but she's basically the executor of his estate. That means she controls everything until they are distributed to the inheritors. That's a huge position of power.
Friendship might not mean much, but a strong possibly ally will.
She can only gain power with the aid of the Inquisition? So I'm guessing her almost becoming the First Enchanter of Montsimmard, her than later becoming the leader of the Loyalist mages, and her personally reshaping what the role of the Court Enchant was all retroactively because of the Inquisitor then?
Neither is likely to happen without the person in question having the support of a massively powerful organization such as the Inquisition backing them.
Or you can just do something heroic that saved the world to gain the ruler's approval like my mage warden, and he was able to become the Teyran of Gwaren. Luckily for Vivienne, she was one of the few people that took part in the final battle against an ancient Darkspawn magister that caused the Blight and processed to threaten the entire world again by ripping a hole in the sky that rained demons.
The only reason Viv can become Divine in Inquisition is because the Chantry is in shambles. Most senior Grand Clerics are dead, the Templars are decimated, Seekers destroyed. They desperately need to support of the Inquisition to regain their former power and will do anything they think will get them into the good graces of the Inquisitor.
Becoming a landed noble as a mage though? Who gains from that? Giving an elf (if Briala is made a noble) land is bad enough. But a mage? Doubtful.
Not so much of "trying to get into the Inquisitor's good graces" if the reveal of her being Divine is a blindside and not something the Inquisitor wanted. That means her getting the role was all behind the scenes manipulations on her part. I know me, and I'm sure a lot of other people, were surprised when she popped up as Divine at the end of the game.
As for you gains from it, it's what a ruler might think a mage might deserve for whatever action they have done for them.