An alternate suggestion that might help - It seems as though you might be over complicating the issue. There are multiple simple backup utilities for Windows, not withstanding your Norton solution.
Admittedly it's a little low-tech, but it seriously fulfills the "offsite" requirement AND removes the network from the equation, to protect against timeouts, lags, dropped VPN connections, etc. while you're backing up.
Using two like-sized external drives, label one "Week 1" and the other "Week 2".
Connect the "Week 1" drive to the computer, configure it to perform a weekly full followed by incrementals, silently in the background. Have your mum remove the drive and set it aside.
On Thursday or whaetver, stop by to raid the fridge, do a little laundry and connect up the drive labeled "Week 2". Doublecheck, make sure that the drive has assumed the same drive letter and your backup software sees it.
Grab and toss the "Week 1" drive on top of the clean shirts and socks in the laundry basket, on your way out the door. Keep it aside, safe and away from danger. This is your mum's `offline storage'.
Rinse, repeat. You've just created a smaller version of what most businesses do every week with their data.
If you've got space to spare or your mum's HD isn't that big and a lot of files don't change that often, feel free to juggle and tweak the times/config a bit if a.) you don't do laundry at home all that much and/or you're comfortable storing more than a week's worth of data on a drive like that.