Hey there! You chose two great examples. For the Dave Malan study, notice how he always uses his hatching to indicate the plane of the surface, giving lots of information about the 3D structure of the subject. For example, around the nose, the hatching indicates the entirety of the bottom surface, as if it were almost like a pyramid or cone structure. I recommend you take a look at the Asaro head, which is a breakdown of the head structure into more simple planes, as I think you'll find it a very insightful reference for faces and simplification. As for the Glen Keane study, he's the master of fluidity and gesture in his linework. If you look closely at his strokes, you'll see how each line he puts down flows throughout its entirety with intent--he never breaks in the middle to start over. In his sketches, he simply puts down the line repeatedly if he feels the need for it. I think this is something you could emulate: instead of going back and forth with small lines or breaking and starting over in the middle of an edge, try putting the line down from start to finish--even if it's not perfect. It doesn't need to be perfect as you can always try the same stroke again, but aim to capture that looseness and confidence in his line quality first. Great job and keep it up! Hope this helps :)

That's what we're here for, to learn! Hope this didn't de-motivate you to continue drawing from imagination, it's a complicated skill, but one that improves with deliberate practice!