Quoting myself again in case anyone is following my little journey...
I think i fixed my overnight battery drain on the iphone 6s. Turns out, when going through settings to try to kill some things one by one, i completely forgot to check on Siri. I'm a "Hey Siri" user (not often but i have it on) and just completely slipped my mind to kill that when trying to diagnose this overnight battery drain issue.
well, last night i finally remembered and disabled "Hey Siri" and happy to report, though cautiously optimistic, that i had zero overnight battery drain after 6 hours (48% at 11pm, 48% at 5am). in previous report above, i had 7-9% drain overnight; ~5 hours of sleep.
now i just have to remember to enable / disable "Hey Siri" accordingly.
Hope this helps others.