: Even with SCS, you should be able to rest as many times as you like in Chateau Irenicus since Imoen is not in your party. I don't know about the Duergar, but if you get the Oil of Speed, there's a trick that can let you get past the Mephit Portal Room without any fighting. Hopefully it works in your install; I haven't tested it in a very long time.
1. Drink the Oil of Speed
2. Run into the Mephit Portal room and attract the attention of all four Mephits
3. Retreat to the previous room and wait until the Mephits are almost next to you
4. Run past the Mephits into the next room and shut the door behind you
The Mephits should be stuck behind the door, and if you time it right, the Radiant Mephit shouldn't be able to hit you with its Color Spray spell, which puts you to sleep for several rounds and has a -3 save penalty, enough to kill almost any class that's not immune to sleep or doesn't have MGOI. You can then take down the Mephit Portals without suffering any attacks.
The Salt Mephit will be mostly luck. The Salt Mephit is in between the Ice Mephit that's fighting the thief, and the group of duergar further north from the Mephit Portal room, so you'll be able to fight it alone as long as you don't rush ahead. The Salt Mephit's Salt Crystal spell has a 50% chance of stunning you for 2 rounds without a saving throw, so, on average, it's a stunlock for anyone not immune to stun. But since there's only a 50% chance per round, it's not guaranteed. You'll be able to attack for one out of every 4 rounds on average. The Salt Mephit will deal 6 damage on average per round when you're stunned, so that amounts to 18 damage for every round you're able to attack it. It has 152 HP, and a Dwarven Defender with the Oil of Speed and specialization but not grandmastery using a nonmagical weapon should be able to deal 300 damage assuming all hits make contact.
The odds of death are fairly low, but it's still very dependent on luck.