I think the comment_formatter.ERROR: format.missing_post_data is being over eager in its reporting. I’ll tweak that for the next release. Thanks for following up about it. I wouldn’t worry about it for now, unless you’re experiencing a visible issue.
There isn’t a formal guide on comment styling. There are a few other topics, e.g. this one. Wordpress themes come in so many shapes and sizes that it was initially thought to leave this up to the site admin. That said, I think some additional guides may be useful.
I’ve put it on my list to add to the WP Discourse guides. I won’t publish that for a few weeks, but if you get stuck with styling in the meantime let me know and I’ll give you some pointers.