Questions about the quality of drawing sessions

[A. K.]

Q.2.  Twisted boxes. So far they're the most confusing to me. Is there any point in looking for a reference for them? Or should I draw them from imagination right away? I can't find anything for a reference. I use plasticine for the bent ones.

A.I recommend you look for semi naked male dancer or belly dancer for women because you want as much skin expose as possible.You can also search for contortionist but it less likely that they will be exposing enough skin to be useful for proper analysis.(You could at leaster stage study the cloth figure)But first it better to learn the underlaying structure (aka anatomy).

Q.3. Gestures. I first drew on a timer (60-120 sec) but didn't notice much result. I switched to thoughtful drawing with thinking about each line and the movement/fluidity. So, should I use timer or not?

Use a timer you are building muscle memory you will not see result right away.What you are doing is building an understanding of how to achieve more in less time will maintaining quality.Basically your building making better choose.But you need some form of accountability that is measure in how much you can achieve in that set amount of time.Dropping the clock is a reason to procrastinate and not to be thinkful so no i do not advise that.

Q.4.Snakes. Сomes out rather unevenly and with a not very believable perspective. Continue to draw or try to draw rings purely to practice the distance between the half-elipses and the smoothness of their turns? I don't understand the sharp turns.

Try to draw half pipe basically you do a curve cylinder and you place ellipse inside it.The key is to understand that when an object turn away from you there is a side of the ellipse that is closer to you and one that is further the one further converge toward the vanishing point which make them bunch up as the go further into distance will the close side is relatively near us so it does converge as much.

Where the ellipse bend the ellipse will bunch up because there is a shift in rotation of those ellipse. 

You should probably learn about foreshorting my explanation is very weak at best.

The key is to understand the view point and where the object point at.You should probably start copying those sheet and try to extrapolate what you learn into building basic scene with ellipse.

The image here only show pipe not half pipe which is a curve cylinder.

Also draw ellipse fully for better understanding

Thank you very much for explanation. One last thing:
"(You could at leaster stage study the cloth figure)But first it better to learn the underlaying structure (aka anatomy)."
I'm not quite sure what you mean by that. You mean the actual cloth figure or clothed models?