To be completely fair to the game you are literally told like 10 times to attend the coronation the moment you enter. All those watchers repeating the same stuff and talking directly with Gortash is kinda hard to ignore. You don't confront Gortash at the coronation the same way you don't confront Ketheric in that room with the goblins. Or, well, you could but it would most likely go badly. Nothing appears as a timed event even if it is cause why would it? Coronations don't tend to last for days :P
It's an unfortunate situation you created. You never met the Duke or spoke to Gortash for some lore so he wasn't sent to the Iron Throne and instead killed off-screen. Not to mention you assaulted the Iron Throne which, with a big warning from Gortash, turned him instantly hostile.
Sure, however unlike other actually time-restrictive moments (such as with True Soul Nere), there was no warning that resting or doing other stuff would prevent us from participating. If we needed to go to the coronation in order to save the Duke, then the game should never have allowed us to even leave the area or warned that things would progress if we did. Additionally, the waypoint to save the Duke consistently showed up in the keep and was there even after we killed everyone and the Journal entry never said we failed to save him. Like, if he's going to be killed off-screen, then maybe communicate that when we hit the story moment where Gortash would kill him (like the game does if you free Nightsong without going to save the Tieflings in Moonrise Tower first).
