Hidden posts have been removed from the posts that are returned by the WordPress route. If you are on the latest version of Discourse, they will no longer show up in the WordPress comments.
The issue with the participants section is harder to deal with. The list of participants that is returned depends on the username that the API request for the Discourse comments is made with. Assuming you are using the All Users API credentials in the plugin’s Connection settings, the participants list will include users who have created whispers.
I am going to add an option to the plugin to allow the participants section to be excluded from the comments area. If you are not able to update the version of the WP Discourse plugin that you are using, you could remove the participants section by customizing the replies_html template.