Your instructor laughed at you for holding your pant leg? I'm sorry to hear that. My friend is a Level 3 instructor & uses that tip to teach her beginning students & more advanced who don't lean forward enough. Eventually you should drop the pant leg once you're feeling more comfortable with your stance.
I would suggest trying your rear binding at a negative angle (angled towards the tail of the board). You will likely find it will aid in your ability to control your turns much better. Also, I don't think it's ever "too early" to start working on flexing & extending the knee to initiate turns. Picking it up early will help to prevent some bad habits & will help you progress a little quicker as you learn to flex your knees in & out of turns, weighting & un-weighting your body - but don't hurt yourself in an attempt to get better or you'll end up regressing instead of progressing. The speed confidence will come as you progress. I think you should develop your skills before being concerned about going faster down runs. One you have the technical skills down you'll find you'll be more comfortable riding at faster speeds since you will have built up your confidence in your ability to handle most anything (if not anything/everything) you'll encounter on the runs you're riding.