Update here. I looked into option #2 (cloning rootfs on Xavier AGX), and it does not appear to be possible once the fstab has been changed. Such a change prevents it from booting properly, and I do not see that there is a way to reset the eMMC to even re-flash the unit. To anyone reading, be aware that these devices are permanently bricked by any ill-conceived fstab changes. There is no possibility for serial connection, cloning the rootfs, nor even re-flashing.
@linuxdev, I took at look at your instructions here, but they depend on the ability to connect to the Jetson via the USB-C in SDKManager, unless I have misunderstood something. Without being able to boot, the Jetson is not visible to either a Linux host connection via USB-C / microUSB serial, nor is the eMMC accessible using any other method.
So it looks like I am stuck with #3, unless someone here has an idea how to save this device.