Comment 31 for bug 1805668

To use arm64 the config.txt should contain arm_64bit=1. You've got kernel=uboot.bin, but there is no uboot.bin on arm64. So what is happening is the pi is falling back to searching for kernel8.img which puts it automatically in arm64 mode. Yes it works, but only because the config.txt contains an error (kernel=uboot.bin).

The pi3 gadget snap doesn't use device_tree_address=0x03000000, nor does the proposed changes to livecd-rootfs last time I checked.

You haven't addressed the use of raspi3-firmware. This is going to overrite any config.txt, bypassing anything flash-kernel does. The two packages are incompatible.