I know that this forum very rarely discuss anything besides proffesional tennis but I need some help.
1. When the ball is short, but really low, how do you hit it most effectively? (It is sliced or something and never bounces high)
Shorten your backswing, and bend your knees more to get down to the ball.
2. Why do my down-the-line shots keep going wide? Is it footwork? How can I improve them?
Partially. If you are right-handed hitting a forehand down the line, your left shoulder should point towards the place where you intend on hitting the ball. Vise-versa for your backhand. Of course, that's assuming you are not using an open-stance (which you shouldn't use if you are a beginner, or intermediate player).
3. Why is my open-stance backhand so weak?
Using an open-stance on the backhand is a low-percentage shot as a beginner or intermediate player.
4. How do I put more... "stick" on my backhand volley?
Strengthen your wrists and remember to "punch" the ball in front of you.
5. What is the best way to handle depth?
On the receiving end, patience and good footwork.
A lot of beginner and intermediate players watch the current pros hitting open-stance shots and attempt to emulate it. More often than not, they fail because they haven't yet learned the proper body mechanics to hit a consistent textbook shot. Once you've mastered the basics, it makes it easier to hit open-stance defensive shots as you improve your game.
My best advice for you would be to participate in group tennis lessons where a teaching-pro can give you advice on what you are doing right and wrong.