The gear shift sensor throttle cut on my BBS02 is too short. I now engage slightly my front brake to cut the motor until the gear change is finished. Work very well for me.
You can lower it by cutting the fork steerer and remove the 2 rings, but it's impossible to extend the fork steerer tube without an additional raiser. You can look for foldable adjustable longer stem like this...but it's ugly ! Replace your handlebar ...
Did you consider replacing the handlebar instead. A dutch or a classic handlebar will look appropriate on your vintage bike and will move the handles higher and closer to you.
10 batteries ? normally a 48v. 500w Greaser battery is made of 5 parallel packs of 13 cells in series( 13x3.7=48.1v, 13x4.2v= 54.6v full) . 10 batteries in series is the way to get 36v batteries (10x4.2= 42v full).
I did it, It's possible and relatively easy to do if you can solder. Open the throttle and switch the wires until you get what you want. Be very careful, because one of the possibilities is full throttle.