so, just to be sure, before i opened a case, i just reset the device using the hard reset switch one more time and tried re-configuring the most simplistic config again. when i did so, everything worked fine (the only thing i knowingly changed is the pass phrases used to generate the AES WPA2 keys). i tried changing the smallest number of things i could from a default configuration, and it now seems stable on both bands.
i am at an utter loss to explain why my 5ghz radio was simply not sending beacons after a time on the last two tries at this... i can tell you that i did do soft-factory-resets of the device (both via gui and command line) before honing in on the final configuration, so there could conceivably be something slightly different between soft reset and hard reset i suppose..
the entirety of everything i've changed includes:
the administrative username / password
the NTP ip address (left it get its WAN address & DNS via DHCP)
the 2.4 Ghz channel to 1 (from smartselect, wireless survey shows me this is ideal)
the 5ghz channel to 161 (from smartselect, wireless survey shows me this is ideal)
set the 2.4 Ghz wlan0 ssid, & wpa2 aes psk
set the 5 Ghz wlan8 ssid & wpa2 aes psk
heck if i know what was wrong... i'll keep an eye on this...