Well, I think you ask the very fundamental question in visual art. And as for any fundamental things, it's a very broad subject that I don't believe I can explain in a reply box alone. But for your perspective question, no, we use both eyes when we draw from life. You shouldn't just use one eye when you draw either (that is if you're lucky to have two eyes of course! If you happen to just have one functioning eye, then well, one is fine!). You see, when we use both of our eyes, the perspective from one eye emerges with the other's and create one composite perspective. That's the reason why if you ever see a 3D movie with that two-colored plastic glasses, you'll notice that when you don't use the glasses, the movie looks like two separate movies combined with a few millimeters offset from each other. In 3D movies, it's the glasses which combine the two perspectives into one, in our case, it's our brain who does that. About drawing from life but the subject is constantly moving, I guess that's the challenge we face every time we draw from life when it's not in studio setting where the model is instructed to stay still. The meaning of life drawing is not that we 'copy' directly from life, but we observe from life, put them into our memory, and then we put the things in our memory onto the paper. So it's not wrong to use memory in life drawing, because memory is a part of life drawing as well. Even though the camera is a perfect image capture device, but it's not a perfect life capture device. As you've pointed out that the camera lacks the ability to capture tones, values, and all as accurately as our eyes do. But that's not all though, when it comes to form and line, the camera does this thing called distortion. Each camera lens (wide lens vs long lens) has a different amount of distortion for a specific purpose. We ideally use wide lens for landscape, and long lens for a portrait, but not everyone knows that. Some people use wide lens for everything from landscape to portraits, and that's where the distortion problem rises. If you ever use a phone (phone usually have a wide lens) to take a selfie, you may have noticed that your face sometimes becomes bigger or smaller than you think it should be. Our eyes are different than that is of camera though, because we can't put the wrong lens in our eyes, they already function perfectly since the beginning. Thus learning lines and forms from our eyes and our visual judgement is better than relying on a photograph. But for the sake of practicality, because we sometimes don't have as much chance to do life drawing so often, photographs actually are not that bad if you can decide the distortion level in those photographs. If the distortion is minimal, then you can use them without worries. The other thing you should look out for in photographs are the editing. Get some photos with minimal editing as well, then you're good to go.