Glad you like the site; you are braver than me to coach 12/13 year old players. Much of your challenges will have to do with the fact that the athletes are just 12/13 years old - You will plant the seeds, but you really won't see the fruits of your labors until they are on to the next coach/team.
In terms of your specific questions:
1. With attacking the ball in front of the body, timing plays a critical part but also the mechanics of the arm swing. If a player 'over runs' the set (is early), then they tend to contact the ball from behind their head. In addition, if the player is late loading their arm swing, or are long and slow with their arm swing, then this will also lead towards hitting the ball behind their head.
To help with the timing, I suggest small ball approach foot work and throws. Get a bunch of tennis balls, or whiffle balls, and have the attacker go through her footwork, accelerating into the jump, then throw the ball with a proper arm swing (just like she was attacking the volleyball) over the net. This teaches the players to throw/swing at the top of their jump and to be able to broad jump through the attack sequence.
Another good drill is the old school hit the ball down into the ground, bounce it up off the gym wall, and it should (with practice) bounce up to be high enough to immediately hit down again into the wall again. It takes a bit of practice, but this is a great drill to teach players to contact the ball in front of their torso, to snap their wrist and to immediately load their arms for another swing.
2. The key for distance setting with control, is quick hands. The faster your hand motion, the truer your target line and the further the ball will fly. Slow hands which rely on a leg push for distance are very inconsistent in length and line. You don't want the setter to punch the ball, but rather get it out of her hands as quickly as possible.
As for back setting, I like my setter to just flick their head back when they back set and watch the ball go over their head for a split second. They still square their hips to the left front target zone, even though they are setting right front. They still have a quick hand release, and keep their point of contact exactly the same. What I don't want to see is any adjustments in body position, just to backset, as this will give it away or lead to inconsistent targeting and height. You do not want to bow the spine back, or lift a leg, or throw the hips forward as this can 'give away' the set to the opposing team, especially the middle blocker, along with delivering an inconsistent ball to the hitter.
Good luck with your season!