The outcome was a bit disappointing. I haven't found any explanation why exactly is the Duke dead, so it feels a bit like an artificial plot device so Mizora has a reason to come to your camp and ask for Wyll's soul.

I tried different save states, but Duke was always dead.

In the end, if you want to have a chance to save both Ravengards, you need to wait for Mizora to come to your camp, refuse her and only then go to the Iron Throne.

Suddenly they hear "Dost thou require an ally... or mayhaps a resurrection instead?"

"Hold on a second Mizora, ladies, I want to get a second opinion.  Withers!  Can you come here?  I want to ask you about the cost of something."

They even revived the Parasite, what a nice bonus!

3:52 She signs Wyll's name on the contract, he doesn't actually sign it. That's a forgery, Mizora, it's not a legally binding contract.

This implies Mizora kills Ravengard in this scenario just so she has a bargaining chip for Wyll.

That “SILENCE, KARLACH!” always cracks me up 😂

shit I wanted her to look surprised, like for once you're actually one step ahead of her

Honestly: I think Mizora has a stronger hand if the Duke is already dead. "Resurrect him" is a stronger argument than "I will protect him", because with the protection scenario, by refusing you at least gives the player hope of saving him while not needing Mizora's help. But the resurrection angle is clearer and gives the player no hope of resolving it by themselves.

If you refuse Mizoras deal but save the Duke anyway and talk to her later at camp you can learn that she still plans to kill Ravengard because it was part of the terms of the original pact. If Wylls pact with her is severed she is entitled to kill Ravengard. She invokes that clause and kills him if you go to the Throne because she offers the second deal. But if you free Ravengard without taking her deal she still is contractually entitled to kill him. She 100% is the one who kills him in the Throne to regain her bargaining power.

In my first playthrough, I didn't ever go to Gortash's coronation, so Duke Ravengard was part of the Gortash boss fight.
I SPECIFICALLY dealt a passive-non-lethal melee attack to him, but he erupts in flames and dies anyway.  The characters even acknowledge that "that blow wasn't supposed to kill him!"  

Mizora shows up all smug and gives the offering of the pact and it's pretty clear that she went out of her way to make sure Ravengard dies so she could have an ace up her sleeve for Wyll's contract.

I mean... Withers is literally a few meters away.

Theo did such a great job portraying Wyll's grief in this moment.

Me with the scroll of Resurrection I stole from Gale: -.-
"I thought the Absolute would have use for him. A general, a figurehead, *something*. Why infect and imprison him, only to kill him?" For plot reasons, Wyll. Plot reasons.

If you don't have Wyll among your companions the Duke is alive and well when you go to the Iron Throne. So literally what this implies is that Mizora finds out that you're on your way to the Iron Throne, panics that she won't have the Duke as a bargaining chip for Wyll and fully kills the guy.

I made him break the pact and then i stumbled across the iron throne. Daddy was still alive (though was very close to being killed by exploding spiders) and saved him without Mizoras help

Player: "Why dont i just walk around literally everywhere to find the dad? OH LOOK. THERE HE IS!!!"
DM: Defeated face, glaring at player "...........................Oh look. Hes dead."

3:08 Arse? Wyll bringing out the real hard-hitting profanity for the devil that murdered his father.

"Mizora, I've come to bargain"
"You've come to die"

What's funny, is that lore-wise, Wyll doesn't even need to get a pact with a devil to resurrect his father. He just needs to go on a journey to find someone that knows how to cast True Resurrection and convince them to resurrect his father. It's definitely hard, but arguably better than taking the easy way out.