Is there any way to monitor total internet usage of all devices connected to the LAN?
Comcast does give usage data (and alarms) but information is delayed and reported only to the nearest GB. Ideally, there would be some method to monitor data in/out of the router in the gateway (Arris TG1682G) rather than monitor each connected device separately. I used Networx on a different router and got good results, but on the TG1682, usage data is low by a factor of more than 10. Other freewares also under-report internet usage. SNMP would be a great protocol but I don't think the TG1682G supports it.
There isn't any way to do that with a Comcast supplied device. Using your own equipment is the only way utilize a traffic analyzer. Here's mine on an Asus router: