How exactly does module_load_include() load a file into the calling script?
It sounds like you're getting confused on scope. There's no "file scope" in PHP. The include/require statements are pretty much the equivalent of dropping the code within the scope that it's called.  

Per PHP's docs on the include statement:


  When a file is included, the code it contains inherits the variable
  scope of the line on which the include occurs. Any variables available
  at that line in the calling file will be available within the called
  file, from that point forward. However, all functions and classes
  defined in the included file have the global scope.


If definitions in the included file are global. For example: 


global $base_url;
define('MYMODULE_CONST', 123);
function mymodule_function() { }


Then they are global until the script execution ends. 

If definition is not global (e.g. $var1 = 'var1 value';) then it only remains within the scope of the include statement.