How can I change a CiviEvent status using Webform Conditionals?
What you are attempting to do should already be handled by the module automatically. Simply omit the status field from the form (leave it set to "Automatic" on the CiviCRM tab of your webform) and the status will be set based on the payment status to either "Registered" or "Pending from pay later" or "Pending from incomplete transaction".

You can see where this is handled by webform-civicrm here in the code.