How do I present the user login form from a callback, and redirect them after login?
You can render the user_login_block on your page. Once the user successfully log in, they will be redirected to the same page on which the user login block was generated (so the same page actually).

To render the user_login_block, you can make use of the drupal_get_form and passing the form name as an argument.

function YOURMODULE_page()
{
    $build = array();

    // construct your build with parts of the page
    // ...

    if (!user_is_logged_in()) {
        // user not logged in, add the login form
        $build['user_login'] = drupal_get_form('user_login_block');
    }

    return $build;
}