I never talk to her, I come into shadow lands from the hill by the creche so get the lantern by killing the goblins before even calling the drider, play Minthara's spider lyre, tell him I killed em all in service of the absolute (which he loves - weirdo), then kill him, free the pixie and get my entire party and all subsequent parties fully barricaded against shadow lands curse that way. Game knows whats happened so I never even get told to speak to her by Jaheira and I avoid going upstairs at all in case I end up inadvertently doing so. Result no fight, no risk of the barrier collapse and no issues with having to free Isobel from Ketherick. I found out about this on my second playthrough - although on my first I managed - more by luck than judgement - to prevent her being kidnapped but some people got killed when they didn't need to be

# Justice for Astarion