form rendering through tpl file
You need rewrite 'form' like this:

/**
 * Implements hook_theme_registry_alter().
 */
function mymodule_theme_registry_alter(&amp;$theme_registry) {
  $form_registry = &amp;$theme_registry['form'];
  $form_registry['function'] = 'mymodule_form_theme';
}
function mymodule_form_theme($element) {
  // I'm not sure that $element has 'from_id' but you could pass it if you need.
  if ($element['form_id'] == 'yours_form_id') {
    return theme_form($element);
  }
  // Else return what you need I guess that better to use hook_theme()
  // and then call theme() function.
}


Look at theme_form() to understand what variables need to pass in form tag.
I don't understand why do you need this. What is the reason to use template file?