Apply a dynamic string to a field before validation
First You should write the these kind logic in validate function rather than the submit function. 

So instead of

$form['#submit'][] = "editorhide_form_submit_handler"; 

you should have 

$form['#validate'][] = "editorhide_form_submit_handler";

Second the first parameter passed to the function form_set_value is the form element and not the form state value so the function should look like.

form_set_value( $form['title'], $fullTitle['0'],$form_state);


I think what you are trying to achieve can be done with a custom module. 

Automatic Nodetitles


  "auto_nodetitle" is a small and efficient module that allows hiding of
  the content title field in the form. To prevent empty content title
  fields one can configure it to generate the title by a given pattern.
  
  When the token module is installed it's possible to use various node
  data for the auto generated title - e.g. use the text of a CCK field
  (since 5.x).