Joining shafts to resist twisting force
You are making this much more complicated than it needs to be. A coupler with two or four set screws is all that is needed.



If the thickness of the coupler is too much then use a thinner tubing and just drill two holes all the way through the tubing and the ends of the two shafts. Then insert a small diameter pin through the holes. The pin can be held in place by using a center punch on each end of the pin to expand it slightly so that it rivets itself into the hole in the tubing. 

For really light duty work the tubing could even be brass and use a pin that can be simply soldered to the tubing. 

All these ideas can be implemented with the most basic tools.