You probably need a new drill bit, the cheap steel in these bars is hard stuff.
You'll be glad you drill that hole, it keeps the throttle assembly from rotating around the bar instead of operating the throttle right. To compensate you will probably overtighten the clamp screws and crack it, then have to deal with that.
You might as well drill that hole.
The throttle should return smoothly to idle. The first thing to check is that the slide is in the barrel right, the slide has a a slot for the guide pin in the barrel. If it isn't right it will stick for sure. The second thing would be a kink or too tight a bend in your throttle cable, the third being the cable not in the 'thimble' right (thanks goes out to 2door), that's the pulley doodad in the throttle housing.
They make cable lube, but graphite or even motor oil can be used to slick up the cable. The inner cable pulls right out of the housing, wipe it with oil and put it back in.
EDIT: In fact it occurred to me that the throttle locating pin sticking out with no hole is binding that thimble in some way, causing the problem. Check that first.
Hope this helps!