The inabilities described in the question don't strike me as either particularly unusual nor as clear signs of a neurological learning disability. They strike me as more likely the passivity and mental shutdown that one frequently encounters in students who, whether consciously or unconsciously, are hiding a sense of incapacity, inability, confusion, etc.
Many students learn arithmetic operationally, as a series of rules for obtaining an answer, without internalizing the significance of the rules or their outcomes. Such a student neither has the habit of associating some mental model (e.g. the circles) with an arithmetic problem nor in many cases is able to relate such a model, posed by someone else, with the problem at hand.
Often students are not asked to consider whether their answers are reasonable (in some qualitative sense). Often they are rewarded (in grading) for performing certain operations correctly, whether or not the outcome is reasonable (with respect to extra-mathematical considerations). So a student who does not think much about what halfway means might easily obtain an answer that is less than either of the averaged numbers. That's not a sign of a learning disability, it's a sign of someone who hasn't learned how to connect arithmetic operations with common sense (in any case, common sense is mostly learned).
As an innumerate student progresses through the educational system, the psychological need to hide inadequacies becomes stronger and stronger, and the possibilities to correct basic misunderstandings become rarer and more remote. The algebra teacher often won't have time, energy, or inclination to teach arithmetic to a student who supposedly has passed several courses that require knowing arithmetic.
It's an extreme case, but once I taught a fourth year university student who was functionally illiterate (how could such a situation occur? This was in the US, and he was the starting tailback on the football team) and largely innumerate. He was adept at coaxing hints from the teacher, and at concealing his ignorance (of which he was fully and quite self-consciously aware). It took considerable care and effort to develop with him the confidence necessary to deal with the problem openly and honestly.
My own suspicion is that far more students have difficulties like this than most teachers care to admit. However, to detect such problems, one needs to interact with such a student personally, individually, and with the confidence sufficient to break down the barriers that prevent honest realization of the psychological obstacles (and the institutional context often makes that difficult or impossible). At the university one experiences that most students who struggle with calculus do so because they don't know basic algebra, even basic arithmetic. (A typical example is that students know that the logarithm and exponential satisfy functional identities, but can't remember if it is $f(xy) = f(x) + f(y)$ or $f(xy) = f(x)f(y)$ of $f(x + y) = f(x)f(y)$ and so guess.) Surely for students struggling in high school mathematics the common difficulty is this sort of inability to relate basic arithmetic to something other than a chain of operations aimed at getting points on an exam.
