1. Yes. But it ranges from painless to pita, depending on which hardware combinations and addresses they use.
2.Not really. As long as they use different Channels and don't need all devices to see each other. Usually you can just disable wireless on one of them.
3.Not sure about where on the Belkin, but most have it on the first page.
4. An unmanaged switch is probably the easiest way to go, That will just add ports and the Comcast with handle the IP addresses, if it is a Hardware router/modem.
My cable modem/router is a Software router (the Brighthouse server does it), so I put a Linksys router behind the modem and plugged the Vonage router and computers into that. Gives me a little better control of my LAN, and Brighthouse only has to handle the one IP addy to my router.