Access to time capsule from main router?
Yes - once you have things set up - Windows, iOS and Mac will all see the Time Capsule even when it's not running the network and your router still handles routing.

Of course it depends on how you set up the Time Capsule since out of the box, it will want to be a router and you could hook things up in a troublesome manner unintentionally. The time capsule has a firewalled connection (The WAN port) that if you use, you have to tear down several settings to allow someone from "outside" the network to see the drive. The Time Capsule has several internal ethernet ports as well as potentially joining the network over WiFi which wouldn't be firewalled.

I would set up the Time Capsule in bridge mode so it doesn't turn on DHCP/NAT, etc and connect to your router if possible over ethernet to one of the LAN ports and not the one WAN port. The ports you want look like this:



As long as it's in bridge mode and you connect over WiFi or a LAN port, everything should be fully functional and all Macs and PC will see the Time Capsule as if it were running the network. It might also function for bonjour sleep proxy and other nice to have items - but that also depends on the router settings.