You won't be able to get the threads 100% perpendicular to the crank. A slight offness and you will feel it when you pedal.

The easiest way, buy a new crankset and this crankset will have replaceable chainrings.

For a shorter person, you need a longer crank, not a shorter crank. A shorter crank you will need a longer leg, a taller person; a child is a short person with a short leg. A longer crank will come up higher. I'm assuming the seat is all the way down. Taller riders have longer cranks because they can generate more leverage plus it's to align their knees and the saddle. If the seat is all the way down and the distance is still too short to reach comfortably, then you need a longer crank that comes up higher.