Is there any way to finish this game without killing the Shriekers? I have hit them multiple times with two different purging wands (neither one of them inert), to no effect. I know that there is something about Vampirism Source but I have not run across it.
If the shriekers at the end of Act 1 are bugged out, then you'd have to stock up on invisibility sources, then chug one for the next character in line to sneak past - and take the left path (central heading straight for Alexandar, right heading for the sneaky treasure crate) and teleport everyone up from this spot: https://dl.dropbox.com/s/rgcex5wk0c7k3v1/sneeky.png
There's a way down from there, you'll just be starting your encounter from a highground choke instead of in the open like this. https://dl.dropbox.com/s/2lxbuk02nn3cpsz/sneeky2.png
Edit: confirmed that you can indeed drink an invisibility potion (chameleon cloak would also work, if you have it) and just walk past the shriekers, then get up and start the fight from said highground position