You're deep in the field definitions there, that's the wrong place, you can't change anything there and you don't need to.
In case of a select, the available options are already part of the form, itform; they should be in $form[$field_name]['widget']...$form[$field_name]['widget']
somewhere.
You
You could also try to use hook_options_list_alter()hook_options_list_alter()
.