You are already protected so You don't need to speak to Isobel and no one will direct you to her. So you don't trigger the fight. Avoid her entirely in fact as you have no need to go upstairs there at all.[/spoiler]
I always kill the drider first but I'm still heading to the Last Light which means meeting Jaheira who will tell me to go and talk to Isobel.
Dark Urge has content with Isobel, its a RPG so exploring everything and meeting everyone is a thing for many players too.
Marcus fight is easy once you learn what to do so no need to totally avoid a good fight and scene and lore and also part of Mol quest.
