There is actually some research on such questions. I am not really conversant with it, but here are a couple of basic points.
First, the 50% rule. Generally learning is most effective when you can complete around one half of the tasks. On that basis, it sounds like the C problems are too easy and the A problems are too hard, right now.

Second, overlearning. When people talk about going over problems until you can solve them very quickly they are talking about overlearning. A music teacher recommended that when you work on a passage that gives you difficulty, don't stop when you can play it right, keep going until it is something that comes easy. He made an analogy with how the body heals a broken bone so that the bone is stronger where it was broken. Overlearning! See
http://senseis.xmp.net/?Overlearning.
BTW, I have doubts about going over problems until you can solve them at a glance. At some point the brain takes a shortcut via recognition. "Oh, yeah, that problem. Black here, White here, blah, blah, blah." The problem is that recognition may rely upon only a few of the stones in the problem. The result in a real game may be, "Black here, White here, oops!"
One possibility when you see the main lines of play at a glance is to consider more lines. See
http://senseis.xmp.net/?GoProblemsTheFudgeFactorAnother idea, which I thought of recently, might be to close your eyes and mentally reconstruct the problem, up to the essential stones. If you can do that, then that should avoid the recognition problem I mentioned.

Another thing, which goes somewhat against the 50% rule, is that when you have a course of study that you are going to complete within a few months (like a semester

), the initial order that you go through the material does not much matter, as long as there is overlearning. That may not matter with the Segoe Tesuji Dictionary, because you can leave the A problem for later. But something like
Life and Death, which you plan to master, you might as well not worry about difficulty.
