My first time doing it, I killed Marcus and a monster dropped her to 0 HP and took her away. I barely finished off the corrupted harpers and tieflings, and Jahiera died. I reloaded an earlier save.
This also makes the big battle on the first floor of Moonrise Towers much, much more difficult. If you struggle with the Isobel fight, you are going to have a terrible time at Moonrise without the aid of the harpers.
Given the massive consequences for failing to protect Isobel, I think maybe it should be easier to do and harder to screw up.
