I know this is going kind of wayward from your initial questions, but in regards to the jerk, if you have an issue with understanding the concept of pushing yourself under the bar after the explosive drive, try working with just a bar at different heights over your head. When I help people understand the concept, this is my go-to drill. You can start at any height with the bar over your head and simulate being at the end of your drive (on your toes). In one swift motion, push yourself -- either into a split or a push -- under the bar without pushing the bar upward. As you begin to feel more comfortable, continue to decrease the height in which you begin -- this will also help you learn to get deeper (bending the back knee) as the weight gets progressively heavier and the bar doesn't jump up as high off your shoulders. The last phase will be to have the bar on your shoulders and work the full jerk. I think you're going about getting your flexibility to improve the right way and this motions will help you "grease up" the shoulder area.
As for losing your snatches forward, it's a result of improper receiving posture, which is related to your stated lack of flexbility in your hips and shoulders. You acknowledge the lack of flexibility, which is good, but don't get frustrated, which is very easy to do. I like spiderman stretches, getting into my squat stance and manually pressing my knees outward while pushing my hips forward for a more upright posture, a figure-four stretch, putting pressure on my knees to further stretch my ankles, PVC pipe pass throughs for shoulders, and rolling on the actual bar for my thoracic region among others. They're really all related -- a lack of mobility in one will lead you to compromise your potential or just flat out miss lifts. If you have Greg's book, there's a pretty good section on the stretches, or you can check out Kelly Starrett.
Continue to stretch and you'll start feeling a lot more comfortable; likewise, do the lifts at a ligther weight to continue getting that flexibility, because you're not going to really get the flexibility required for the movements without doing the movements. Having the weightlifting coach around will help you, too.
I hope this helps you in some way.
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.