How to get value of Textarea in node add form?
Shortly put, you need to do the following:


Use hook_form_alter, the way you do already
Use hook_node_submit, to update the field_title field on the node object, before it gets saved. In your hook implementation you will have access to both $form and $form_state, which should carry your custom textarea submitted value. Even if the nid is not there, your changes should still get stored in the new node.


Hope this helps!