Access variables populated in one page function for a different page function? Always null
First off, Drupal is bootstrapped every time a new page is requested. Even if both the pages  would be using the same menu callback, the global variables would be reset when passing from a page to the other one.

What you can do is using session variables, passing variables through the URL, or using a cache.
Passing variables through the URL would not use for complex variables, as there is a limit to the URL length accepted from browsers; using a cache would be a solution if the cached data should be available to more than one user, such as to user having the same rule; using session variables is the easier solution when the data to share are only for the currently logged-in users.

As for using the same callback, there is an example given in the hook_menu() documentation.

  function mymodule_menu() {
    $items['abc/def'] = array(
      'page callback' =&gt; 'mymodule_abc_view',
    );
    return $items;
  }

  function mymodule_abc_view($ghi = 0, $jkl = '') {
    // ...
  }


The menu callback would be used for http://example.com/abc/def, http://example.com/abc/def/222, and http://example.com/abc/def/333/aaaa. In the first case, $ghi and $jkl would get their default value; in the second case, only $ikl would get its default value; in the last case, none of the variables would get their default values.

That is true also for the form builder, with the exception that the first two arguments would be $form, and &amp;$form_state.