woo! i'm an uber hack!
so, basically, i couldn't figure out what the issue was, but the entire block causing trouble here was "If it is an anonymous post, don't give a link to their profile"
and since i dont allow anonymous posts, and i don't care (that much) about profiles (until there's an upgrade)... i just commented out lines 921 through 956.
so now, all the threads work again for viewing and replying. the only issue is all the user names are text, and not links. ...but i can live with that for now! hopefully the upgrade (later this... year?) will just swap out all the bad for the good, anyway.
regardless, thanks for your assistance, gliderdad!