Hi, if your receipt of mailing is within the 72hrs you’re ok. Once it’s mailed within the 72hr time frame, you’ve met your obligation and remain in honour and what happens in the postal service is out of your hands. Any delay within the postal service is outside of your control, therefore the receipt of mailing is your proof that you fulfilled your obligation to respond within 72hrs of receiving the presentment.
Can’t answer the other points sorry