Yeah, do lean to about the same angle as the trees growing on the hill.
With regard to her wanting to trot down hills:
Nice to hear that you are getting out on such long trail rides.
Gidget probably trots downhill out of both laziness and lack of balance, which is due to not haveing the abdominal muscles developed that hold her back, against gravity. These are the same muscles that a horse uses in collection, so it will be a worthy goal for the two of you to work on developing these. Gidget is a bit downhill by conformation, if I remember correctly, so she has to work a little harder to get her legs under her.
Not only would I work on getting her to stop and back up on the dowhills, but I would do it on the flat too . It's important that she be respectful of the bit and not lean on it or you hands. If you have this on hte flat it will be easier to get it on the hill. I use a three count process of stopping my horse cleanly, and I mean I literally am counting, ONE, TWO, THREE, in concert with an exhalation. Like this:
YOu get ready to stop her (in YOUR mind), take a deep breath and stop YOUR body (stop moving with her), sit up really straight and imagine your feet just dragging along the ground. You get kind of heavy. This is count one and you begin exhaleing. Count two, exhale more, partially , and do it audiably, and get heavier and start shutting down your hands (stop followoing her mouth and tighten your hand on the rein), Look for your horse to show that that she is preparing to stop. Think "and THREE! meaning STOP!" and count three , exhale fully and fREEZE! YOur hands stop, your body stops and you are like concrete. Think of your elbow being linked to the spot on the ground where you said "three". If your horse keeps moving, you do NOT release them at all. YOu hold the brick wall in your hands. They should literally "bounce off" that brick wall and take a step back, til they come back to where you went "concret", 'cause that is where you laid down the stop line. Count one and two prepare, count three is stop, here, now!
If the horse stops nicely on three, give a slight release, then ask them to shift their weight backward , as if they will back up, then give a big release and ask them to go forward.
Do this on the flat. Then, as you get better at it and your horse gets the signals the prepare her for the coming stop, start doing it on a hill. in the beginning, do it not far from the top of the hill, before she has descended much and gained speed, so that you can keep her in control better. As she gets better, do it in the middle, and just at the bottom of the hill where she would like to run out the last bit.
It is hard for a horse to back up a hill, but it builds good muscles, so do this excersize and even ask her to back up the hill a step or two.
I agree that your friends should be more "friendly" and wait at the bottom fo the hill for you when you are doing these sorts of training things. Better yet, you be in the lead and you back G up and thus make all of them back their horses up , too. It would be good for them, too.