How much bandwidth on the dedicated internet link?
Off the top of my head - I like the idea of you acting as an ISP to the stores and deploying a BR1 or Balance 20x per store. That way each store has their own isolated LAN that can be configured as they need it to be and you then provide the internet to those devices. You could choose to do so over both wired and wireless connectivity back to your core network VLANs, each VLAN could have a captive portal on it (there is a max limit on captive portals per device but I can’t remember what it is). You could also sell 4G data to the stores as backup / failover or for more bandwidth.
At the core I would be considering the SDX or the new (not sure if its released yet) B580X for the additional throughput and the ability to add a 4G module directly to the device.
Yes. You would have a Shop VLAN that is presented to the stores over ethernet or wifi. The BR1 or Balance 20x (or whatever router they have there doing NAT) would then access that shop VLAN and you would set a daily / monthly data limit on the captive portal for each store.