My partner and I just took out Gortash last night as part of us finishing up the last story quests before ending this playthrough. Sadly, Duke Ravenguard was nowhere to be found, despite the market for the quest being in the middle of the keep and Mizora telling us he'd be at the coronation.
We also looked for him at the Iron Throne but he wasn't there and that, I think, is part of the problem and what caused him to essentially die off-screen.
You see, we tend to complete every quest outside of what we consider the mandatory main story quests before continuing on with the story. As such, we didn't go back and attend the coronation with Wyll after Mizora mentioned the Duke would be there, mostly because it didn't appear as a time-restrictive event.
So, we ran around, doing every single side quest we came across in the Lower City (except the ghosts because that was so buggy and un-fun that I just refused to play it), including completing the Iron Throne. We managed to open all of the cells and free all but one of the prisoners (a Gondite who sadly just got unlucky) and the Duke was nowhere to be found.
Then, with all of the side quests behind us, we went to the keep to confront Gortash, disrupt the coronation, and save Duke Ravenguard...except the Duke wasn't there.
We left the keep to talk to Mizora and she started lamenting the fact that Wyll's father was dead and we freaked out. We had no opportunity at all to save him. I can't tell if this just bugged out and we got screwed but it really feels like we were robbed of our opportunity to save him and get the best ending for Wyll because the story logic triggers couldn't handle us doing something out of order with no warnings that we needed to do something in a specific order.