It looks like there's a lot of room for improvement. The only thing I can really see you focusing on is keeping an arch in your back. You aren't loading up your hips and hamstrings and you aren't bracing your abs. Sometimes you aren't using your lats to pull the bar to your body. The lack of ab bracing and using the hips is likely whats contributing to your back pain. Your knees are so far forward because you aren't loading up your hamstrings. Eventually you'll want to learn to create tension throughout your entire body during the lift.
I recommend three movements: goblet squat, romanian deadlift and lunges.
With goblet squats, take in a deep breath and brace your abs hard every rep (poke your sides to make sure it isn't soft). Push your knees apart with your glutes, which will also cause your hips to push forward. Focus on tightness in the erectors, abs, hip flexors and glutes throughout the lift. This is where you learn to spread/screw the floor. When ascending, lead with your chest.
With romanian deadlifts, brace your abs hard, tighten your erectors and use your lats to pull the bar to your body. Keep tension in your hip flexors throughout the lift. Load up your hamstrings. Your knees will be slightly forward to allow you to use your quads. Balance the weight between the balls and heels of your feet to ensure you're using both your hamstrings and quads. While keeping everything tight, lower yourself only to the point that you can maintain tightness even if starting off with a ridiculously short range of motion. Don't sacrifice muscle recruitment over range of motion. Eventually you will incorporate spreading/screwing the floor but worry about that later - master the ab bracing first to protest your back.
With lunges, keep your entire torso tight and make sure you feel tension in your hip flexors and glutes when doing the movement.
The main goal is to get you to learn what the movements feel like. It'll take a few months of working on these lifts. You can eventually progress the romanian deadlift to full ROM deadlifts. Take your time to progress and don't rush the process or you may have to relearn it. If it's too much at once, focus on one or two things each session and progressively add in more cues. Doing a simple linear progression like 3x5 is fine. Start off light and don't be afraid to eventually go heavy. It won't look great at first but you'll have to strive to maintain technique which will help you build the muscles you want.
There is no best stance to save your lower back. You have to learn to use the above muscles effectively to reduce risk of injury and increase performance. Your optimal stance for performance is your own preference and you can make tweaks to figure that out after learning the above. Hope that helps.
Edit: I forgot to add that with the lunges, you learn to pull your hips forward with your hip flexors. Combining this pull forward when using your hip flexors with pushing your hips forward and knees/feet out with your glutes results in spreading/screwing the floor.