Sorry, the function does need to return 'form' and you must remove the menu_get_object() from the submit function and use an alternative method to get the node. As you now have access to the node id in the submit fuction, you can use node_load()
I have updated the answer showing how to add a hidden field to the form. This value should now be available in the submit handler. This should allow you to load the node without using the menu_get_object() function in the submit.
It would depend on where your form is being displayed. Is the form being displayed on a node page? If so, you could add a hidden form element containing the node id, then it should be available in the submit function.
Then I suspect your problem is with your call to menu_get_object as the current path in the ajax request is system/ajax so `menu_get_item' will not work in an ajax call. You are probably going to have to send an argument in the url that will allow you to load the appropriate node.
