On several occasions, I have noticed that enemies can be sighted (just within the line of sight/fog of war of the characters) but will not attack, even when fired upon (example: Yxunomei was killed by one of my characters--a thief--by arrows, and she didn't fight back; she was just on the border of my thief's fog of war--in the dark, but not--but would not attack but took damage and was eventually killed). Is this a glitch in the game, and if so, should the patch have fixed this? (speaking of the patch, after I installed it, a message appeared, asking if I wanted to replace the existing IWD file with a new one; I entered "no"--should I have said "yes"?)

Yes, I've seen that, too. Only a very few times, though. You have to be standing in exactly the right place. I've tried a couple times to make it happen on purpose, but could never manage it. However, I didn't try very seriously. I think it's a problem with the line-of-sight code. You can see the foe (just barely) but they can't see you. Shouldn't happen, but it does in rare instances. Dunno if the patch fixes it. But maybe I haven't seen it after installing the patch. Hard to say.

About the patch: This is a joke, right? The instructions explicitly say you should answer "Yes" when asked about replacing files. In fact, the instructions say it's best to answer "Yes to all". Since you didn't replace some file (or files) you haven't really installed the patch. One thing the patch does (just like most patches) is replace small files that have errors in them. For all I know, it's the only thing the patch does.