I am not that smart, I am afraid. I still do not build my own Sachs bottom ends/gearbox into casings, for instance. I still screw things up. I have just been around a long time and learned a lot from folk more skilled or experienced than myself. So many folk do not take the trouble to watch what the experienced do, question it to understand, and remember. Folk like Malcolm Thomas, Ian Hopkins, and so on, used to freely help those who suffered failure on rally, they still do, but with some favouritism against those who are in the same failure as three times before! They are not present as a free RAC service to those who feel no need to learn. Your thanks to these guys is to take the knowledge they freely gave, understand it, and pass it on. Secondly it is to admit you do not know, if you do not and invite advice. There is nothing worse than wrong advice. Someone somewhere will have had a similar problem, even if it might be a different machine, or even in a differing scale of engineering. Its what draws the oily together and marks them from those who wish to, and enjoy, clean handed showing of cars. Its a different hobby, I think.
Anyway to the specific questions.
The drill was to attempt to break up the hollow plug of metal jammed into the threaded hole of the crank. THis took out metal, leaving the threads contaminated, but pretty mush unsupported by interstitial material stiffening the structural strength, but also blocking the inward passage of the tapered cutting tap. The unwanted metal was fortunately softer than the crank so it was possible to feel the start of the original thread. One chance to get it right or effectively the engine was scrap. Many would argue I over complicated the process, but I prefer to be cautious as I am not an engineer.
It was not the thread that was wobbly, well not after a couple of turns. It was the taper surface itself. Clearly a high spot, or malformation.
I think the woodruff key had been beaten into the slot with blows to its surface. This effectively flattening the key slightly, raising the edge of the key slot slightly? I suspect the key-way was either originally to tall. or the swarf made it to tall, if the swarf was really under the key. I cannot swear that it was.
I can remove and change the key, its not of tough material here. Its a location device, but also sacrificial should there be a seizure of such violence that the inertia of the dynostart might shear the crank. Better the taper face slip and the key shear off. This is more evident in the Sach 200, with a more massive dynostarter and also not unknown on the cardon shaft drive gears, if the rear wheel or chain has locked up on a KR200. Harry Conners tunned his Schmitt to rev up to some 7000 revs. The problem came when he needed to slow down quickly in an incident caused by someone else. The inertia of the dynostart turning was too great for the crank, which sheared through. In this case the woodruff key did not sacrifice itself on the slipping taper. Better that it had,
The crank I cannot exchange so I dressed the area around the key way slot to remove any high spots. Now, keyless the taper fits together without any movement. So I feel that with a correct sized (standard) key I have a repair, and that I can pull up the taper by using the 6 mm threaded bolt, as it should be done. It should not need to be excessively tight to stay put. I have not a torque setting to hand, but I guess about 65 lb ft. For me that is wrist tight.