I'm not an expert spinner, but have been spinning for 10 years or so.
Setting the twist.... very subjective.
I will wash my yarn after spinning, but it is more to make sure any dirt is out of the yarn plus to check on the dye (if I've used dyed roving) to make sure the dye doesn't break (bleed).
One wash with liquid detergent .... I'm not gentle with my yarn when washing. I'll swish it around and bunch it up, letting the twist redistribute through out the yarn.
One rinse with hair conditioner ... again, I'm not gentle with it.
Then outside to whirl it around to remove excess water and again redistributing the twist.
Hang to dry without weights.
If your singles are over twisted, why not try plying two singles together?
Roll each single into a ball, put each ball in it's own container (like a bowl) to help keep it from bouncing all over the room. Attach the singles on your leader and spin your spindle in the opposite direction that you originally spun the singles.