If I have understood the Cellular Data setting correctly, it means that the phone will fall back to cellular if the current WiFi connection has no route to the Internet (i.e. a bad hotspot of some sort) - something I was under the impression that WP8 already did. Thanks for pointing out that this is now a setting in W10, which was disabled for me.
Now, the bad news is that enabling this setting changed nothing, nor did enabling OneDrive to upload over cellular data connections. Uploads are still extremely slow, error prone, and consume battery (relevant to this thread). Back to USB it is.
Since you are now happy (?) with the battery life after the hard reset, has anything in the power usage distribution changed? For me, 60-70% of the power is consumed by the Display, followed by System at 25%. 80-90% consumption is foreground apps, with Edge being the top app with over 50% of the total. These numbers have been fairly stable for me - are they very different from yours?