It's honestly not that hard. I was completely out of spell slots because I also had no idea of the impending battle for the reasons you mentioned. That said, I only had to reload once with a single person next to Isobel and everyone else on the balcony outside with Marcus blocking the door.
It's not "poorly designed" and having multiple save slots can help alleviate some of the issues you're having. I always run with 1 AS, 1 QS, and two regular files for that reason. Maybe more AS slots would benefit you.
As far as practical advise, I'd also like to remind you that you can throw healing potions to heal someone. That helped me quite a bit in this fight.
