custom comment fields on CPT

Non-CPT Comments

Regarding the first issue: you’re not returning anything if your conditional is false. To simplify your code:

function debate_comment_fields( $fields ) {
    // If conditional is true
    if( is_singular( 'debate' ) ) {

        // Do some stuff to $fields
        // Return $fields
        return $fields;
    }
    // If conditional is not true?
} 
add_filter('comment_form_default_fields','debate_comment_fields');  

You need to return the default $fields if your conditional is not true:

function debate_comment_fields( $fields ) {
    // If conditional is true
    if( is_singular( 'debate' ) ) {

        // Do some stuff to $fields
        // Return $fields
        return $fields;
    }
    // If conditional is not true?
    return $fields;
} 
add_filter('comment_form_default_fields','debate_comment_fields');  

This will ensure that the default fields are returned for non-CPT post comments.

CPT Comments

The second issue you’re running into is that, for your CPT comments, the comment form isn’t sending name and email, which are required. There are two solutions:

1. Disable name/email as required fields globally
2. Filter preprocess_comment to allow CPT post comments to bypass the required-field check