From what you describe and the things that have been tried, I'd probably pull the throttle body and give it a good thorough cleaning and inspection to make sure nothing is sticking including the IAC stepper motor.
Pay close attention to the throttle valve/butterfly that its not rubbing the inside of the TB bore when opening and closing due to the shaft having moved or something worn. I suspect that may be where the problem lies and if so should show some rub marks in the bore when closely inspected or maybe try to stick when operated slowly by hand. You might even try pushing on the shaft very lightly to one side then the other side while moving it to see if the valve plate tries to stick or touch the TB bore.
While the TB is off Id also start at the throttle control on the handlebar and clean, reset, lube it and make sure the cable is properly adjusted for about 1/16 - 1/8" play and that it has no kinks or pinch points and moves smoothly through the full range of motion.
If you adjusted on the throttle stop/idle screw it will probably need to have the TPS reset/recalibrated too.
I hate to say it but if it appears the problem is in the TB as I suspect, and it can't be easily corrected then the TB may have to be replaced.
I'm in no way saying that's definitely what's wrong but it sorta sounds like it and that's how I would approach finding the problem if I were trying to fix it.
Good luck and let us know what you find.