The first problem is that you're trying too hard. Use the router/switch to share everything and it should all fall into place.
Connect this way: cable to Comcast modem, ethernet from modem to router's WAN port (that's very important!), then each wired computer (and the AirPort Extreme Base Station) to a router LAN port. You can connect a wireless access point to one of those LAN ports too-I do that to provide my own wireless network.
The only complication is the fact that you have to tell the Extreme to be an access point. You do that by unchecking the "share a single IP address" box on the Network tab of the Admin Utility.
Does connecting things this way resolve some of your issues?