No, I appreciate the attention Jethro!
1) The URL doesn’t have to end in .ics, no.
2) Right, no password required for this file.
3) WEBDAV isn’t enabled in any way, but I’m pretty sure HTTP is fine for this… again, it used to work in iCal, and still works in Sunbird.
My hunch is that there’s some sort of foreign character or whitespace issue hanging iCal up; I just can’t figure out what!