Yes, all motor kits come with a locking clutch lever.
It works like a normal lever, but when you have it pulled in you can push that button down with your thumb to lock it.
Note: It locks about 2/3rds of the way pulled in. This is where you want to fine tune you clutch adjustment to roll completely free, but engage the motor when you pull the lever a bit so the spring pops the button up and lets the lever go back all the way.
In short, you lock it in Neutral with that button.
As far as lever angle goes, everyones hands are different sizes and sit at different heights.
For me on that bike about 65° works best to put the lever on that first joint from the fingernail.
If you just snug up both levers and then sit how you ride you can use one hand to rotate the other hands lever to be in the perfect location for you on that bike, then tweak them down.
To me the handlebars are the most important part of an MB.
At least in my experience nothing fatigues me more than crappy grips and levers in the wrong place so I NEVER use those crappy hard kit handgrips and use at least BMX foam grips, they will fit over the throttle barrel.
In my pic examples I was able to use the foam and leather Giant Bike original grips, but you won't find that many bike grips as they won't go over the throttle barrel.