the mannquinization is pretty good ,i like how you use line to convey depth.Just watch for proportion.combine shape with form :)

you are really going in a right direction , just a few pointers. just use the shapes as it is dont manipulate it so much , (except for arms and feet in which u have to use a tapering cylinder ) for example for the rib cage in the 2nd and the 3 rd drawing . lastly work on your head proportion , for that just visually measure. here are some of the figures that I made right now using the same reference images

I am noticing a common habit of yours, you are drawing the heads way too small. I run into this problem myself when drawing gestures since I don't have much information with just a circle. I would put in a bit more information of the head first with the mass of the hair to have something to judge other relationships based on. Also, when asking for critiques check the mark for "Help Request" or people will have to go directly to your topic and cannot reply on the main post.

I wouldn't worry about any conventional horizon lines or any vanishing points for that matter - if you are drawing from these sorts of references, and just working on the figures themselves (that is to say, you aren't drawing them as a part of bigger scene). The photos have the figures so close to the picture plane in deep space that any real convergence of any curved or straight line would be pretty minimal IF you are plotting a vanishing point. Plus, you would be tracking which contour and cross-contour lines are parallel and converging, and which are not. With an organic form, that's pretty hard and overkill imo. What could be useful is working on foreshortening. Which is just a form staying at the same distance from the picture plan, but changing orientation. https://youtu.be/R60e9_ofV68?t=600 This whole video is great, but Marshal Vandruff show's what I'm talking about here pretty nicely and concisely.

Hi, hope this can help. It seems like you do have a good understanding of form, but struggle to simplify it on the figure. Try to make it even simpler by representing the torso with just two boxes (robo bean like) and the limbs as cylinders. From there, you can create a smooth transition betwen hard and soft forms. It appears that the organic curves that the muscles create are distracting you from creating a solid form, so try to make the forms without too much distortion, the curves that proko does in his examples are subtler. Pretty much what's left is proportion, take your time to measure a little bit more your reference. About the question, I think the most important part of this exercise is understanding the perspective of the large masses of the body and how they correlate each other, rather than the perspective of the whole scene, that's some adavanced figure drawing exercise, but there are clues in some poses tha can give us clues for where to eyeball the horizon line. For example, when you have a model estanding over a set up or props, like boxes or a carpet, they can give you information about the hight of the horizon line. You can also stimate if a part of the body is estanding straight, for example; on the first pose, I deduce that the ribcage is in a neutral position, in addition to that, the head also looks like is not tilting, so now I have 2 forms that CAN be standing straight (they might not be exactly like that, but are close enough) so I track the vanishing points from the head (almost strainght) and the shoulders (from the closest to the farthest) and more or less when they align horizontaly, that's the horizon line. To internalize this whole process takes a lot of time and requires many drawing sessions just drawing forms in perdpective and rotate them. Sorry if ts was a log comment but I tried to be as clear as possible, have a nice day!