Reaching out as I don’t know what else to do.
The product page on the store has my email address? 
that message on the mac means its dp phy has given up. You might be able to get it back by rebooting the mac.
Moshi is the best brand of usbc to dp but others have had success with club3d usbc to dp.
The club3d dp to dp cable out of the kvm into the g9, is that 3 foot? if not you might try fibbr brand fiber optic cables. The 2m and longer cables from that is fine and lower loss than club3d.
“but the cable works fine by itself” so this is the wrong way to think about it.
“rightsholders” want to know about anything that could eavesdrop on the DP bus so repeaters tend to show up in the DP topology. The kvm’s design goal is to be invisible. How to think of it then? One long analog signal corridor from the gpu to the kvm and beyond, through the next cable, to the monitor. So your usb c to dp cable has to have high enough SNR to put out a signal not just to a monitor 1 inch away but through several feet of copper cable.
The g9 is particularly picky.
You might be able to enable dsc/240hz on the neo g9? That might possibly work with the macs? macs do have fleeting support for dsc in some scenarios because they painted themselves into a corner – some of their highest end displays don’t even work reliably without it because the port out of the laptop barely works.
Anyway, other customers have a similar setup and it works. It is possible its something with the kvm, you can mail it in and I can test it with the macs and odyssey g9 that are here, but there is variation from device to device so that’s not a guarantee.
I am hoping that telling you that combinations such as cable A plus cable B hinge on it working/not working because the kvm doesn’t have a repeater in it as a design goal and that might be the secret to it working.
The other secret to it working is that if you had a marginal cable on the other side of the usbc cable it will remember and not retrain very hard. Hence rebooting the mac and also unplugging the adapter from both ends since it saps power from the bus. Doesn’t hurt to power cycle the KVM, too.
Yes, you are literally rebooting a cable in some scenarios like usbc to dp. There’s processing silicon in there too! Also, this sounds stupid, but right side up usbc is not the same as upside down usbc. Was playing with a moshi cable today that worked better “logo up” than “logo down” I kid you not.
Moshi is a better brand than club3d for the dp 1.4 to usbc adapter, and you may require fiberoptic low-loss cabling from the kvm to the display in order to work with the mac sadness.