I think you could do it. Pressurize the crankcase keeping air blowing out the holes at all time. Grease up the bits and taps to catch the breakthrough pieces. Go slow and methodical.
Know in the back of your mind that you will possibly loose a chip or 2 to the cavity. Maybe flush the case with thin oil or spirits when done. Hopefully you have the large sump plug to catch what you might loose to the process, if any.
My 850 has 3 Helicoils an i remember reading somewhere that many owners of later engines also had them leading to the thought they were put in at the factory from new.
Before you drill poke something through th hole to check where the crank is positioned. If its behind that hole move it before working on that hole. Don't fit replacement screws that are to long for the same reason as the may foul the crank.
I don't know if that applied to "ALL" Commandos, at least not the early models, but helicoils seem to have been fitted in the oil pump stud holes and primary case from, I think, around 1971-72, and later, the 850 barrel base stud/bolt threads were normally helicoiled (maybe some of the 850 crankcase joint threads are, too)?
My 69/70 didn't have a helicoil in it anywhere. I replaced those primary to crank bolts with studs and nuts, red locktite into the crank. My bike actually came with one of those crank holes already enlarged to 5/16". They were supposed to be 1/4" as I remember.
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