The demon killing Corinna if you use the fake branch is part of the cut scene, so you can not move until it is too late.
She is a good source for charms and occasionally has spell books, but if you don't have a recent save before that point, I wouldn't worry about it. If you have a save after giving her Mpenzak's fake branch but before the summoning, there is a
Fix for giving Corinna a fake Talking Tree branch.
