"There is no info on the path or URL I need to supply for the CRON job to initiate the backups in any WP DB settings that I can see." -- That is not needed, WP will does a pseudo cronjob
"So I am guessing the plugin SHOULD be setting an internal WP Cron job to be initiated by a page loading" -- Correct, as I was saying above.
Your case is tricky because it is not just WP-DBmanager not working? Jetpack as well? How about trying to post it in the JetPack support, you might get answers as I think more people look at the forums then this.