Installation/gateway question: Xfinity cable/internet customer with owned Arris SB6141 modem downgrading to Internet Now 200MB or 300MB

Our family has been a Comcast Cable/internet subscriber for decades. To reduce monthly cost, we recently decided to drop cable TV and downgrade to Xfinity's 200MB Internet Now plan.  We just received the Now installation kit and Arris TG1682P gateway and are trying to decide how/whether to reconfigure our home network for the Internet Now service.

We currently own two Arris SB6141 cable modems (one spare) and use a TP-Link Archer C9 router which is configured for a complicated mix of static IP reservations and DCHP clients which includes NAS, printers, and media servers.

I've got the following questions about how to coordinate termination of our old service and the best way to configure the new service.  Call to customer support have resulted in confusing and conflicting answers:

1. Can I / should I try to use the Arris SB6141 cable modem instead of the refurbished Arris TG1682P gateway?
2. How does the Arris TG1682P compare to the SB6141 in terms of performance and features?
3. As the TP-Link Archer C9 router is already configured for WiFi networks and devices, should I place the Arris TG1682P in bridge mode and connect to the C9 router?
4. Xfinity customer service agents offered me a 300MB plan for $35/month, but somehow I was switched to the 200MB Now plan for $45/month. Can I switch to the 300MB plan? 

Thanks in advance for any subscriber or CS help!