Okay, starting a new comment. My brain has been backward and thinking about this wrong.

So, you have a KVM that only supports HDMI. Have you confirmed that this KVM can handle a >1080p signal? If it can't, then all of this is moot.

Also, you say it's a Mac 5,1 - a 2010 "Nehalem/Westmere" Mac Pro? What video card are you using? Because AFAIK, none of those came with a video card that had an HDMI port. Does your video card support >1080p over HDMI?

That also isn't in your diagram. Make sure to include all the bits.

So what we want is:

💻 (HDMI) -> KVM Switch (HDMI) -> (something) -> mDP-to-DL-DVI adapter -> 🖥️ 30" CD (DL-DVI)

Assuming the video card in the Mac Pro and the KVM switch both support HDMI 1.3 or higher (the minimum version to support >1080p), AND you're using HDMI 1.3-certified HDMI cables…

Where I have (something) try an HDMI to miniDisplayPort active adapter - plug the HDMI port into the KVM switch's "monitor" out port, plug the mDP-to-DL-DVI adapter into this device's mDP female port.

.

^{And now I'm closing the 23 browser tabs I had open investigating this.}