Moving the upper control arms from the unibody structure to the K-frame was more about ride quality than anything else. General idea being that any road vibration that gets picked up by the upper control arm has to travel through the rubber k-frame iso mounts before being picked up by the front stub frame. A secondary benefit might have to do with production cost by eliminating welded in inner fenders.

No clue what, if any, value is added by the diagonal braces from the cowl to mid fender. For starters they are thin wall tubes and ya can pretty much bend them with your hand. The fenders are thicker and it's not like they rattle around with those bars. The metal inner fender which is mostly a splash shield likely adds more stiffness to the fenders than that brace does.

The k-frames in these things are massive, boxed and plenty stiff but they do have weak spots. The worst being where the rear mounts attach and if yer gonna crack one, that's where it's gonna happen. Adding a plate there couldn't hurt. I would tend to agree with statement that the front bumper reinforcement adds more structure to the front of the frame rails than the rad core support does. It's one seriously solid hunk of steel. As you intend to delete that, adding something to core couldn't hurt. As for the idea of running some tubing from the front of the frame to the firewall, the weak link in that is the firewall. Only way I see that having any real value is if it goes through the firewall and ties in to a full roll cage.

If you've been around these things for a while you've probably seen than full tilt road race Doba that occasionally pops up for sale. Owner strikes me as more than willing to discuss the mods he's made. There's a couple hints about front frame mods in the first vid here:

https://bringatrailer.com/listing/1977-chrysler-cordoba-3/