Re question #2, Amazon uses UltraDNS as a backup and seemed to be relatively unaffected by today's attack.

Re question #1, check out PagerDuty's reliability page here: https://www.pagerduty.com/features/always-on-reliability/

Namely "Uninterrupted Service at Scale - Our service is distributed across multiple data centers and hosting providers, so that if one goes down, we stay available."

It seems fair to expect them to have a backup dns too, but I am not an expert.