Is there a way to return/throw a user error in the UI on an After Submit script? I know you can do it on Before Submit.

I don't understand logically why you would want to let something get saved and stored, and then throw an error as opposed to stopping it on beforeSubmit.

Because there are values that Netsuite would need to create first when creating a record that aren't available when it's in Before Submit that need to be accessed.

Did you already try throwing in the after submit? Logically you've now put bad data in the DB, so it seems like a bad process. I would maybe try to emulate whatever information Netsuite needs to make after submitting and validate against that before storing.

Not the perfect solution... but check the end of this thread