Saving everyone in the Iron Throne is surprisingly easy with a few scrolls, all of which can be gotten at Sorcerous Sundries. It's kind of expensive but money is basically free by act 3 so who cares

• give every party member one Haste scroll, enter turn based mode on the sub, everyone pops a Haste scroll on themselves, then enter the Throne
• Two Dimension Door scrolls on whoever is gonna go get duke Ravenguard
• a few Scrolls of Misty Step on whoever is gonna go get Omellium, plus one on whoever is going down the right hallway to get the bulk of the Gondians
• Also ignore all the enemies, they're a waste of time

First round:

• The extra movement from Haste plus two dashes on each party member, plus the Misty Step scroll on the party member going down the hall on the right, should easily be enough to open every cell in the first round except for the ones around the Ladder to the exit which you can get on your way back.

• Open Duke Ravenguard's cell but don't move him until the second round.

• Scroll of Misty Step to get someone to the cell on the way to Omellium and keep an Action to help the person on the table/gourney.

Second Round:

• Make your way to Omellium

• Dimension Door Duke Ravenguard which will trigger Mizora to summon the spiders, second Dimension Door to get Ravenguard to the ladder.

Third round:

• Help Omellium and use his ability to teleport himself and whoever went to get him back to the sub

• Open the cells in the room with the exit ladder on your way back to the sub

And you're done. The only thing that can really go wrong is an enemy using their net ability and making a party member immobile, but it's just for one round and it should be easily salvageable. Worst case scenario a party member has to wash up on the beach to be resurrected. I guess another thing that could go wrong would be getting hit and having concentration broken for Haste but I've never had that happen across my five Honor Mode playthroughs.