Best practice for cashier empowerment is: If it's under a $20 price difference, change it. *I tell mine that if they feel uncomfortable, they can call me, but unless it requires my supervisor number you're more than empowered to make the decision yourself.
Best practice for lines getting backed up: If lines exceed 1 + 1 and you can't clean it up by speedweaving, call for backup. Backing up can vary by store, though. At my Ultra Low Volume store, the GSA/GSTL always answers backup calls except during peak/busiest sales hours. We utilize our front end team first; so if the cart attendant can backup cashier, pull him or her to do that before calling for the sales floor. Guest traffic is hard to predict, but you can call for backups as often as necessary, sometimes that'll be every 20 minutes, sometimes it'll only be 1 or 2 times per day.
As far as getting tasks done..... Your guests come first. Guest service comes first. Plan your day accordingly so you (or somebody) can work reshop/zone/dollarspot/candy early in the morning or late at night (when it's slow enough to do so.) You should be driving for conversion, cashier speed, fast checkout, and great service first... Finishing any of those tasks you mentioned should rank second behind guest service 99% of the time!
Hopefully that helped. There are a lot of knowledgable GSTL's and GSA's on this forum, so feel free to browse around or reach out to us 🙂