Unfortueny I can't answer these questions:
1. Where does it get the current timestamp from
2. What maths does it use to add to that to get the next backup date
3. What has to run/load/fire to initiate the backup process
because all this are part of WP inner workings and I have to really dig in (which I can't due to time constraint).
Maybe Googling will get you more help than from me, https://www.google.com/search?q=wp-cron+debug
