cant flash a recovery using terminal mode while unrooted... and what are you trying to do the update using? also you shouldnt need to unroot to update no matter what method you use, the unroot option is only meant for those who plan to return their phones and dont want to get caught up with the whole warranty voiding aspect, and also for those who just dont want root functionality, anyone worried about their kids breaking their phone, anyone who just doesnt know enough about what to do and what not to do. all you have to do is install the recovery. if fastboot mode doesnt work for you you will probably need to manually force the driver, my tool has a link to my guide that will explain, it would have to be in that mode where the light is on and the green android guy is stuck on the screen though
now as far as the problems you are having... is the phone just rebooting randomly or is it resetting itself while tethering because even on the charger tethering uses more power than it can take in. also if you didnt already, kill the connection optimizer it will mess up tether and really just make your internet connection more unstable in general which in my experience its pretty unstable to begin with
my advice if you want to update though
reroot
use my tool to run a terminal install of stock recovery, try to run the update. if that doesnt work and you dont wanna just try to use one of the copies of stock b09 we have around here well i think ur sol. installing the recovery should work, but i cant promise the ota update itself will