It looks like a 25-pair phone cable. You could use it to carry Ethernet if the other end is in a useful location.

You can do this but it will suck. May as well just put a USB WiFi dongle on your PC. If you can run a cable to your computer— or at least between the APs— do it.

The top is for splitting a phone line 8 ways. Cable splitter, and then a network patch panel

If you put your gateway, switches, routers, access points, etc. on a UPS? Sure. Assuming whatever equipment comcast sends the service to your home also has power… if it’s a wide enough outage, it may not.

Ubuntu is usually my go-to Linux for servers