Having separated network without router mode

I believe you are correct that Guest network is not supported when the Orbi is in Access Point (AP) mode:

https://kb.netgear.com/000061277/Which-features-are-disabled-on-my-Orbi-router-when-it-is-set-to-AP-Mode 

Is the Fritzbox present because it is a combination modem/router device?

Does ths "new internet connection" replace the existing internet connection, or does it result in two simultaneous internet connections?

My intuition is that the desired network diagram is incorrect.  With the Orbi in AP mode, all devices connected to the network will receive IP assignments from the Fritzbox, including those connected directly to the Fritzbox, all of the Orbi components, and everything connected to the Orbi (ethernet and WiFi).  Everything will be in 192.168.178.x  There will be no 192.168.179.x network at all.

I suspect the only way to separate Guest WiFi from the rest of the network is to have the Orbi as the primary router.