hey zebzfree! nice stuff here. that last vine charcoal drawing is nice. and your sketchbook pages are great too. keep drawing with those straight lines! they're great for measuring. it keeps shapes relative to each other.
here's a quick rundown on drawing figures from your head.
if you are trying to get better at drawing the figure out of your head, just build the figure with basic forms. do lots of gesture drawings to get the feel of the rhythm of the body. rhythms of the whole whole figure/pose, then the rhythms of the muscles within that figure. then memorize basic proportions, build with cylinders, blocks, spheres, cones. the key to making these things sing though, is perspective. perspective is the most overlooked thing in drawing figures out of your head. if you draw every figure like it was a building (in perspective), then you will have no trouble drawing them from your head. just remember your basic forms.
here's the thing, if you just memorize lines/contours of the figure, you can't move the figure around 3 dimensionally in of your head. if you memorize lines, then you are just recalling shapes/lines that you've drawn from a regular eye level---this is very limiting when drawing out of your head. if you think of movie costume designer sketches---most draw their costumed figures very frontal/flat, because they are only looking at silohuettes and shapes). Like I said, this is limiting because they memorized one basic view.
Remember, everything you see conforms to the laws of perpective, including figures.
try to think: Space, Form, Light.
i remember iain mccaig saying that he would draw the perpective plane(the Space) that his character is going to stand on, then draw from the feet up to the head (like an architect builds a building, from the ground up). it makes sure that your character is planted. then he'll figure out the Forms. and then once he knows the landscape of the Forms) he's able to light the face by their planes (walls of the building.)
Also remember that drawing is drawing. Drawing architecture in perspective is like drawing people in perpective. Complex forms are complex forms no matter how massive or tiny. And to draw a complex form you must understand the what basic shapes and forms it is composed of.
Learning to draw is more about learning to see.
I hope this helps and I hope I wasn't too confusing. Good luck!