I've never had a problem having stones stay in the sling. As far as the spin goes, a sling already puts some pretty serious spin on the projectile.  I've had probably half a dozen cement projectiles rip apart in mid air just from the spin. (Most likely already weakened from previous hits, but it's a pretty cool sight either way. )   

That being said, no harm in experimenting.

No.
You don't need magnets.

Plus magnets would completely bugger up the release. Actually make it pretty dangerous as you'd have no clue when it would release.

If your missiles are not staying in your sling - make a better sling
Traditional split pouch slings - in my opinion - are NOT suitable for beginners.