Hmm, that didn’t work (I don’t think CSS written on the parent page can affect anything inside the iframe anyway?).
Tried a few other things (adding the following to the CSS, changing the width back to 100%), and suddenly the comments are showing up again.
.fb-comments span, .fb-comments iframe {
width: 100% !important;
}
I’ll mark this as resolved for now, though this seems to be an intermittent issue.