WiFI connection from modem/router to Time Capsule

So that would mean connecting the MacBook Pro to the modem/router by WiFi and the modem/router to Time Capsule by WiFi, while maintaining iMac WiFi connections to TC.

It is not possible to locate the TC next to the router and use an Ethernet cable.

You don't have to locate the TC next to the router. Ethernet can be run 300+ feet or 100 meters with zero signal loss, so you should be able to locate the TC just about anywhere that you want. My TC is located probably 60 feet or more away from the main router.

If you cannot run the Ethernet cable, consider using a pair of Ethernet over Powerline adapters. These products send an encoded Ethernet signal over the existing AC wiring in your home......so the theory with EoP is that you already have the wiring in place, you just need the adapters.

EoP does not work well for everyone though, so if you want to consider a pair of adapters, it would be a good idea to understand the store's return policy before you buy......in case things don't work as well as expected, and you have to try another brand.

Any computer electronics superstore will have a selection of powerline adapters from which to choose. Ask the sales people what brand that they would recommend for your particular needs.

Unfortunately, the TC cannot connect to the modem/router using only Wi/Fi, so best not to spend much time thinking about that....since it is not an option.