Damn. Do you at least have control over pressure-sensitive brush opacity, brush definition and shape? If not do switch to SAI. It may not be perfect but I know it at least allows you to control that.
Anyway here are a few observations that may or may not benefit you:
-make sure that the facial features are aligned with the overall shape of the face. I've seen you do it perfectly right sometimes, but when you try to simplify the shape of the face to two strokes (which I advise against since it doesn't take into account the difference between jawline and cheeks), you often end up with a nose and mouth that don't match the position of the chin. Spend more time during the sketching phase to make sure the face looks like a solid object, and DON'T HESITATE TO USE REFERENCES as long as you don't just trace them.
-also remember that even stylized faces (I mean, those that look attractive) usually retain the same general relative proportions as real human faces. Read up a little on that. Also one thing you might need to do more often is to draw the fold of the upper eyelid that's a little above the eye itself.
-Pay attention to the relative length of limbs. The thighs should be roughly as long as the body, the fully-extended arms should reach down mid-thigh, the shins are about as long as the thighs, and the body is about three to four heads tall. Knowing that you should be able to make your characters look more convincing.
-you always draw arms that are very spindly and shoulders that are very small. If that's your fetish keep it up, but otherwise try to fix it cause it's a little freaky.
-I use a trick during the sketching phase to make sure all the limbs and segments are well-connected and proportionally correct: I sketch the whole character's silhouette almost without lifting my pen from the tablet (and of course I try to keep my hand relaxed). That makes one big squiggly line which I find very convenient to work with and get the outline of the main bones and muscle groups right. It also works wonders for the hands (not for the faces though). Of course this works better when the opacity of the brush is pressure-sensitive. Like so:

This is a shitty sketch but as you can see it already looks more or less three-dimensional (kind of like a very messy wireframe model) and I think it's one possible way of approaching Kishimoto's way of drawing bodies during the sketching phase.
-be careful with foreshortening. In your latest comic page there's one arm that's all weird because of this. The technique I just mentioned is especially useful for this because it forces you to keep all the segments connected instead of just guessing where stuff is supposed to connect. But overall using references is the surest way.
-Jeff Smith has great penmanship; all of his lines look bouncy and elastic, but are all perfectly connected, which is what you should strive for (at least during the clean-up stage); the thing is that lines are primarily stylized shadows, so their thickness depends a lot upon the roundness or sharpness of the shapes they define: sharper edges usually mean thinner lines unless there's supposed to be a strong contrast behind it.
Anyway, if you want to be proficient in several styles, you need to try a realistic style first. Not necessarily with only values of grey; you can do it with only lines, but like, find a few models and sketch them up in your sketchbook, like a ten poses within 15 minutes, using the techniques I mentioned. It will at least get your hand used to it.
As for collabs, I suppose we could pitch each other ideas via MP, then we'd both work on the same picture, each of us would do one small step of the work, pass it on to the other who would do the next, and so on until it's finished. That way we'd be able to correct each other.