Agree with you need to give some Epsom salts. Or something sulphur/mag rich.
Depends, anywhere from 70-110. You'd of gotten more had you filled the net more
So, you grow the nodes during the stretch (before pre flower) and fill in the sides. Flipping at 75-85% of the net (depending on the strain)
Also do some light lollipoppoing once pre flower has stopped. It'll help bulk up your colas.