Hi,
Each cron plugin have his own "frequency" option.
The cron plugin could be called every hour but each plugin would be independent. So the "low stock" notification could be triggered once a week if you set the right frequency value.
About waitlist notification, the plugin has been updated not so far (in a recent release, the 2.1.1 or the 2.1.2) and the SQL query has been improved.
(b.product_quantity > 0) OR (b.product_quantity = -1 AND b.product_type = '.$db->Quote('main').') OR (b.product_type = '.$db->Quote('variant').' AND b.product_quantity = -1 AND (c.product_quantity > 0 OR c.product_quantity = -1))
It reads values of the variant but if the variant stock is "unlimited" (-1), it reads the value of the main product.
Regards,