Problem with drupal_valid_path
I would guess the problem is that drupal_valid_path() is expecting a system path, and you're passing it an alias.

Fortunately there's a function to determine the system path from the alias, drupal_lookup_path(). I would have thought a simple change to your code like this would make it work:

$target_path = drupal_lookup_path('some/path');
if(drupal_valid_path($target_path, FALSE) === FALSE){
  // create 'some/path'
}