override a default .tpl.php file in a custom module

to basically use a template file in a custom module use the hook_theme_registry_alter(&$theme_registry) like this

function mymodule_theme_registry_alter(&$theme_registry) {
  // this will override the default page.tpl.php by page--mymodule.tpl.php located at the root of mymodule
  $module_path = drupal_get_path('module', 'mymodule');
 
  $theme_registry['page__mymodule'] = array(
    'template' => $module_path.'/page--mymodule',
    'type' => 'theme_engine',
    'theme path' => $module_path,
    'render element' => 'page',
  );
}

Display errors - WSOD

The bootstrap override the values you could set to display error ad the reporting level. To be 100% sure everything is printed out when something fails in a blank screen (wsod), make index.php like this:

Properly render a field

Drupal offers a few (sometimes complicated...) solutions to output a field. Lets take an example with a date field.
To display the full rendered field: field_view_field()