Hello RK
I'm quite comfortable with the sense of completeness in this passage:
"Of course she was very busy all day long, but whenever she had a little spare time she sat down to spin. Her distaff turned of itself and her spindle span by itself and the flax wound itself off; and however much she might use there was always plenty left."
At the time of speaking, the using and being-plenty-left were complete.
For the connection with "may", I would change the passage as follows:
"Of course she is very busy all day long, but whenever she has a little spare time she sits down to spin. Her distaff turns of itself and her spindle spins by itself and the flax winds itself off; and however much she may use, there is always plenty left."
All the best,
MrP