1. The problem you've mentioned about arm angle might be because of the elbow. I did the pose myself and the elbow isn't low like that(Unless the model was different. I could be mistaken.) For me it took up the middle of my arm. It takes up the whole thing.
This could be a model thing, I'd love to see reference just to make sure.
2. The problem you're seeing with arm length could be solved by shortening it about a hand length. It's not that far off. You did pretty well.
3. Her back currently looks a little flat. Remember, people have form. If you have to, draw the action line. It usually follows the spine and it can give more life to your drawing. The line around the waist with the jeans is a little flat as well. So is the butt. Don't be afraid to add meat.
4. Her legs are too long or her head and torso are too small. She stands about 9 heads tall at the moment. She should be standing about 7 and 1/2 heads tall if you're going for more realistic proportions.
NOW, this could be a thing with the model, but I don't have the reference in front of me, so I'm just going off general basics. She could have freaky long legs. I know people who do.
5. In my opinion, and it could be wrong, I think the fingers are a little short. The palm and finger are pretty close to the same size. Again this could be a model thing. I'll let you use your judgement on that.
Remember, cloth doesn't shine unless it's silk or satin or things like that! Denim absorbs light and stays a darker shade. You aren't going to get a bright return off of them like you do with the skin.
One thing you're doing right is remembering the position of the light source. Way to go!
Keep it up, JJJ!