Let's try to dump pstore from the device when it's trying to boot then.
Wait for the device to reboot after being on the amazon logo, immediately on the next boot up, go into recovery mode. Then, enable ADB.
You can dump pstore with the following adb command (adbd should run as root in recovery):
adb pull /sys/fs/pstore
Hopefully there should be some logs in there which you can attach here, if not, keep retrying until you get something in there. If you are 100% nothing is being dumped to pstore, just let me know.
As a side note, I am using the kernel source and blobs from the 7.3.2.7 firmware, so I'm not sure why it's not seeming to boot on later firmware.