Couple of bugs

First plugin did not work until I deleted the site key and secret from contact form 7’s integrations page (/wp-admin/admin.php?page=wpcf7-integration).

Second, it saves erroneous backslashes in the options, like Invalid captcha error message because it uses the raw (backslash-escaped) $_POST variables and passes them through sanitize_text_field() which is meant for HTML (it escapes into HTML entities). You might want to use wp_strip_all_tags() but you at least need to do stripslashes() (and probably trim()) on them first.

After working around those, the plugin operates as expected.