Altering user_admin_account and user_register_form

I'm pretty new to Drupal and I'm trying to build a Drupal 7 custom module to alter the user listings table in admin/people. I would like to display the first and last names of the users. I've already added the text fields to the user registration form in admin/config/people/account-settings/manage-fields and I've managed to store the user input in my database table. I've added the extra fields to the user listings table but can't figure out how to retrieve the values from my database table and output them.

Here's what I have so far in my .module file:

<?php

/**
 * Implements hook_form_FORMID_alter()
 * @param $form
 * @param $form_state
 * @param $form_id
 * @return $form
 */
function wdt_user_fields_form_user_admin_account_alter(&$form, $form_state, $form_id) {
  //Adding User 'First Name' field in the 'user_admin_account' form
  $name_header = array(
    'f_name' => array('data' => t('First Name'), 'field' => 'wdt.first_name'),
    'l_name' => array('data' => t('Last Name'), 'field' => 'wdt.last_name'),
  );

  //Place field
  array_splice($form['accounts']['#header'], 1, 0, $name_header);

  foreach($form['accounts']['#options'] as $key => $value) {
    $query = db_select('wdt_user_fields', 'wdt')
      ->fields('wdt', array('first_name', 'last_name'))
      ->condition('wdt.wdt_username', $key)
      ->execute()
      ->fetchAssoc();

  }

  $form['pager'] = array('#markup' => theme('pager'));
  return $form;
}

/**
 * Implements hook_user_insert()
 * @param $edit
 * @param $account
 * @param $category
 */
function wdt_user_fields_user_insert(&$edit, $account, $category) {
  db_insert('wdt_user_fields')
    ->fields(array(
      'wdt_username' => $account->name,
      'first_name' => $edit['field_first_name']['und'][0]['value'],
      'last_name' => $edit['field_last_name']['und'][0]['value'],
    ))->execute();
}

I'm pretty sure I'm doing something wrong when it comes to the $query part but I can't seem to find anything online.

In addition to this, I've used the 'name' from the 'users' table as the primary key in my custom table. I'd like my table to be updated when a user is deleted, so the first and last name in my table are deleted too. I'm not too sure how to do this. Does anyone have any ideas?

If anyone could help me with any of these questions it'd be great. Thanks in advance.