<?php
/**
 * Plugin Name: Rearrange Comment Fields
 * Plugin URI:  http://wordpress.stackexchange.com/a/207449/26350
 * Author:      Birgir Erlendsson (birgire)
 * Description: Support for rearranging the comment fields: 'comment', 'auhtor', 'url', 'email' and 'submit' through the 'wpse_comment_fields' filter.
 * Version:     0.0.1
 */

add_action( 'comment_form_before', function()
{
    if( class_exists( 'WPSE_Rearrange_Comment_Fields' ) )
    {
        $fields = apply_filters( 
            'wpse_comment_fields', 
            [ 'comment', 'author', 'url', 'email', 'submit' ]
        );

        $o = new WPSE_Rearrange_Comment_fields;
        $o->set_fields( $fields )
          ->init();
    }
});

class WPSE_Rearrange_Comment_Fields
{
    private $html       = [];
    private $defaults   = [];
    private $fields     = [];

    public function set_fields( array $fields )
    {
        $this->fields = $fields;
        return $this;
    }

    public function init()
    {
        // Default
        $this->defaults = [ 'comment', 'author', 'url', 'email', 'submit' ];

        // Check for defaults
        if( empty( $this->fields ) )
            $this->fields = $this->defaults;

        // Hooks
        add_action( 'comment_form',                 [$this, 'display'],                     PHP_INT_MAX );
        add_filter( 'comment_form_field_comment',   [$this, 'comment_form_field_comment'],  PHP_INT_MAX );
        add_filter( 'comment_form_submit_field',    [$this, 'comment_form_submit_field'],   PHP_INT_MAX );
        foreach( [ 'author', 'url', 'email' ] as $field )
            add_filter( "comment_form_field_{$field}",  [$this, 'comment_form_field'], PHP_INT_MAX );       
    }

    public function display()
    {
        // Display fields in the custom order                   
        $html = '';
        foreach( (array) $this->fields as $field )
        {
            if( in_array( $field, $this->defaults ) && isset($this->html[$field]) ) 
                $html .= $this->html[$field]; 
        }
        echo balanceTags($html);
    }

    public function comment_form_submit_field( $submit_field )
    {
        $this->html['submit'] = $submit_field;
        return '';
    }

    public function comment_form_field_comment( $comment_field )
    {
        $this->html['comment'] = $comment_field;
        return '';
    }

    public function comment_form_field( $field )
    {
        $key = str_replace( 'comment_form_field_', '', current_filter() );
        $this->html[$key] = $field;
        return '';
    }

} // end class
add_filter( 'wpse_comment_fields', function( $fields )
{
    // Re-arrange fields
    $fields = [  'author', 'email', 'comment', 'submit' ];

    return $fields;
} );