My odroid-xu4 is back in service. After leaving it offline for a few days to see if any new details came to light, I decided to give another crack at bringing it to life. I hooked up full monitor and keyboard and tried booting into my attempted 3.10.105 recovery, but it just hung halfway through boot (no obvious indication as to reason for hanging).
I then booted onto the freshly built SD image, which came up on the 3.10.105 kernel fine. I arch-chrooted to my eMMC and figured I'd give a full update a go, since the kernel had been revved at least twice since my previous attempt. Upon boot, it hung waiting for single-user login, complaining about some mounts. This odroid happens to serve NFS for me, so I went and removed all NFS boot processes from /etc/systemd/system and then rebooted and viola! it came up fine.
I almost posted here, but then remembered people said they could get their odroids to boot, but they either wouldn't stay up or would hang on reboot. I can confirm the hang on reboot - if I reboot my odroid, it won't come back up until I physically disconnect power and then power it back up. However, that issue aside, my odroid has been running 4.9.13-4 for a few days now and is stable. I haven't tried turning NFS back on yet.
I have two odroid-xu4's, one is actually my primary home router. Running arch as your internet router is living dangerously..
and having been bit a few times before, I now have the second odroid-xu4 that gets all updates/reboots before I upgrade my router. Since rebooting cleanly in the case of a power out is a critical function of an internet router, my router odroid-xu4 will continue running 3.10.105 until I see the 4.9.x kernel get to a stable point on my second one.
But just wanted to add to the information available to maintainers that might be keeping an eye on this thread. With NFS disabled, my odroid-xu4 appears to be stable running 4.9.13-4, as long as I don't try to reboot it.