That was a fantastic, fantastic post, much better than what I could muster myself. I think the arguments you present as your reading of Hume are correct. Which gives me very little to room to disagree so far. Let's see how to add or comment, and proceed:
we can't raise the question of external objects — because Nature — but we can look for causes of the belief we're stuck with. — Srap Tasmaner
Yes. And notice a difficulty here, nature has made this issue too important to leave it to us to decide if objects ("body") exist or no, this, we take "for granted". Yet the thing taken for granted is what paves the way for Hume to ask, essentially, well what reasons do we have to believe in the continued existence of these objects? It turns out that the reasons we have (or the ones he gives) are not nearly as good as we would like to have.
You seem to be more methodical than me, so I'll add what I think I can contribute to, by way of agreement or disagreement, and perhaps not mention a section which others might find crucial, if so, they can bring it up.
He goes on to mention (in part iv) that the perceptions we have of objects are actual perceptions. It makes no sense that we should say that the perception feels different from the object, whose impression gives us the idea of it.
Then, concerning external existence, Hume states:
"The paper, on which I write at present, is beyond my hand. The table is beyond the paper. The walls of the chamber beyond the table. And in casting my eye towards the window, I perceive a great extent of fields and buildings beyond my chamber. From all this it may be infer'd, that no other faculty is requir'd, beside the senses, to convince us of the external existence of body." (pp.190-191)
This is pretty clear and one would even say, a "naive realist" view of the world. But he is quick to point, we to take into account several important facts (three in total), of which I will mention only the first, as it looks to me the most important one:
"...properly speaking, ’tis not our body we perceive, when we regard our limbs and members but certain impressions, which enter by the senses." (p.191)
Which is true, and reminds me of Russell's comment that, strictly speaking, a neurologist is not looking at a brain when he studies it. He has a perception of something, which we call a brain, it's not as if the neurologist looking at a brain, is much different from us looking at our bodies, both are perceptions of "brains" and "bodies".
He points out that some of the things we attribute to external bodies, on minimal consideration, turn out to be internal affections, heat and sweetness and colours, etc. How far do we take this? It's not trivial, but we must at the very least allow the opportunity of contact with an object, to gain an impression, but for Hume, it's much more than this minimal consideration.
To end this post, he reiterates:
"[the senses] give us no notion of continu’d existence, because they cannot operate beyond the extent, in which they really Operate. They [the senses] as little produce the opinion of a distinct existence, because they neither can- offer it to the mind as represented, nor as original. To offer it as represented they [the senses] must present both an object and an image. To make it appear as original, they must convey a falshood ; and this falshood must lie in the relations and situation: In order to
which they must be able to compare the object with ourselves; and even in that case they do not, nor is it possible they shou’d, deceive us" (pp.191-192)
As I understand it, if the senses represented "originals", objects as they are, we should be able to then compare these objects to ourselves, which would make them "external and independent" from us, as he says in p.190, 3rd paragraph.