I honestly don’t see a reason why it would hurt if there’s nothing wired to the breaker. And it feels safer than leaving the contacts exposed since the metal cover is open on those slots. But I’m not sure if that’s the proper way.
As for the wire, I think it’s more so quality control. If it’s new and you bought it then you know it’s high quality and if it doesn’t work you can warranty your own work, but you can’t warranty old equipment. But this is another one where I can’t see the harm in using it if I am replacing a short strand of wire and it is the right length. Like I said, it’s type NM cable so it seems to be equivalent to modern romex wire, just a different brand.