Comcast’s slim silver lining from Q3 was the performance of its year-old MVNO business, which the operator is using to boost broadband usage and in turn leverage as a platform for selling video services. It’s one big, convoluted loop that has certainly caught the attention of rival Charter Communications. It begins with Comcast’s Xfinity Mobile service, which gained 204,000 subscribers in the third quarter to bring its mobile footprint up to 1.8 million subs riding on the Verizon mobile network. But Comcast has sought to offload traffic via its prolific installed base of 19 million Xfinity WiFi hotspots, which automatically switches Xfinity Mobile subscribers onto Xfinity WiFi hotspots when in range. Reducing Xfinity traffic going over the Verizon network reduces…