The Netgear WGR614's WAN port should be connected to the Cable Modem's ethernet port. The Modem's USB port is *NOT* used.
The PC, and any other devices going online, connect to the Router, either via one of its four LAN ports or "wirelessly".
Therefore Q1 - you cannot dispense with the Cable Modem, the Router connects to it.
Q2 - When using a router, you must connect by ethernet, not USB. Unless the PC doesn't have a network adapter installed, there is no good reason for using USB in any case, ethernet should always be used in preference to USB.
Also, ensure you follow the correct procedure when swapping connections to the Cable Modem:
Power everything off, including the Cable Modem which must be left without power for three or four minutes - this is IMPORTANT so that it loses the association between the existing connection (recognised by its MAC address) and the IP address that it has allocated to it.
Connect a network cable between the router's WAN port and the Cable Modem's Ethernet port. Connect a second network cable between the PC's network port and one of the Router's LAN ports. Power on the Cable Modem and, when fully booted with all lights displayed, power on the Router. When the router is fully up and running, boot the PC. The Cable Modem will recognise a new connection and allocate a different IP address to it.
If you now type the Router's IP address into your web browser you should be able to open its configuration pages to set it up for wireless networking. You do not need to be connected to the internet to access the router's configuration pages from your web browser.
As far as connecting to the ISP is concerned, "Cable" uses the very basic setting to connect and get its addresses by DHCP. In the Basic settings of the WGR614, it asks if your internet connection requires a Login and you select "No". That's basically it.