It would be helpful to know your location ,maybe a member could guide you through some repairs.

HOLD THE PRESSES!I BELIEVE  I HAVE DISCOVERED  YOUR PROBLEM! IT'S SABOTAGE! BY THE OWNER! May I quote you?" I'm in the habit of putting in an additive each time I fill up the tank" Most additives are petroleum distillates ,(AKA-diesel fuel/KEROSENE).seeing that you never really use all the gas in your tank,if your habit is to add the same amount of additive each time ,you have over many fill ups ,slowly turned the fuel in your tank from gas into diesel fuel,WHICH IS HEAVIER THAN GAS ,it settles to the bottom of the tank at smoother highway speeds and you get a slug of it in the cylinder the sparkplug can't ignite it ,it then leaves unburnt into the hot exhaust pipe which does ignite it causing the bike to backfire ,and die,killing the motor.As the fuel level drops in your tank the concentration becomes stronger ,this explains your the shorter length in time between your problems on the road, and the fact that in the end the bike wouldn't start at all. " When the bike sits for 15 min. and cools off then it will start right up"No when it sits the oil in the fuel settles to the bottom of the carb fuel bowl and the gas moves upward toward the jets allowing the bike to be restarted .If you pull the sparkplug and it has black soot on it ,that would be an indication ,stock motors usually run lean and clean plugs. drain all the present fuel from the tank and carb bowl ,replace with new (I' d use 91 octane this time) and use a new or cleaned sparkplug.SKIP USING ADDITIVE ,IT'S A WASTE OF MONEY ( unless your putting the bike in storage ,then use sta-bil ),as the bike doesn't require it ,and they are for one time ,not continual use.

We have over 7000 members world wide ,most here in the states ,where are you located ? state and nearest city? someone could be close enough to lend a hand.

.  I have one neighbor who rides.  He uses the same mechanic and doesn't even change his own oil so he is useless.

That's a good attitude. You'll probably be okay on your own. The CATCH, spotting the Additive thing, good job..
Now, has your cam been scorched by the mechanic? Might wanna look at the valve adjustment. If it's much different between intake and exhaust and the exhaust is looser, that could be bad news. I would want to know before I took it home. The mechanic who said he knows how to work on it might be paying for the parts.

You don't have to remove the tank to change the sparkplug ,you only have to remove the chrome cover on top of the head,and use the sparkplug socket in the tool pouch.