First: My earlier post was in response to what you had asked at the end of yours -- about how many admirers of HPL first read him at 35 or older (though I think the specific age was intended to be just an indicator of maturity rather than a definite lowest age). What I was indicating is that there have been plenty such, from various fields, and various cultures.
Now, given the definition from About.com (and, for that matter, the rather lengthier entry on plot at Wikipedia, which cites from Aristotle, Freytag, etc.), I would say that, on the whole, Lovecraft was actually very good with plots. I tend to agree with Peter Penzoldt when he says that Lovecraft's stories were "nearly always perfect in structure" (FDOC, p. 69). And as for a series of incidents, their structure and relationship, and their effect (change) on the protagonist (as well as other characters at times), I would say that these were there in abundance. At times they are not explicitly stated or "diagrammed", but they are nonetheless there.
I will also venture to disagree with both Wilum and S. T. to a degree. In his earlier writings on the art, he did indeed express a strong interest in plot, though later it came to be of secondary importance to creating an impression or mood. Nonetheless, he viewed the best way to get there as closely involving plot, in most cases (some particular types of story, the "pure fantasy", as it were, are less reliant on this), as can be seen in his "Notes on Writing Weird Fiction", which states that one should write two synopses: one outlining the events and action in order of occurrence, the second in order of narration, before proceeding on to writing the story in even a rough draft form. So obviously, however he put it second to weaving the mood, it remained quite important to him.
On your second point, I would say that it was less "horror" that he wished to evoke, but rather "terror" , albeit "horror" is there in plenty; e.g., the shoggoths in At the Mountains of Madness are primarily horror (though also with some ontological terror involved) whereas the final climax of the thing beyond that second mountain range is an evocation of terror. This is carefully prepared for by the references throughout the novel of the plateau of Leng and Kadath mingled with the Dunsanian references and the dream imagery, gradually breaking down the accepted laws of time and space. Above all, this is a realm where all such structures break down, and this leaves us with no anchors to moor us, hence the mood here is one of true terror rather than horror.
On the point that it has to be done in a "temporal" state -- he wished to evoke that feeling, yes, to capture that moment; but in order to do so, he presents us with a series of incidents which gradually build toward such a moment, rather than simply attempting to capture that moment itself. True, he sets out by identifying that moment, but always with a great deal of reservation; presenting the reader with (as Henry James said was so important) a mystery which the rest of the tale is required to resolve. Hence, also, his reliance on the "terminal climax" as everything which comes in between is meant to build toward that one final moment (Poe's "singleness of impression").
This use of plot can be seen as early as "The Beast in the Cave", albeit there his youth causes the tale to be somewhat pedantic in presentation. But by "The Tomb", it is much more carefully crafted and all extraneous material is winnowed out, so that by the end the narrator has undergone an intensely drastic change, in becoming possessed of his ancestor (whether by this we mean an actual case of the spirit of that ancestor possessing his descendant, or his obsession with the tomb and its inhabitants -- particularly this one -- causes him to take on that personality).
I think, too, that the concept of "plot" as described here may mean different things to you and I. I would agree that many of Lovecraft's stories are not "action-driven" as much as "knowledge-driven"; in fact, there is a distinct element of the detective story to many of Lovecraft's works as the protagonist strives to piece together the actual picture, generally to their detriment. But I would argue that he does indeed utilize a series of incidents which gradually cause a shift in his character(s), usually leaving the protagonist horrifically isolated in one form or another (the most extreme perhaps being the Outsider), but in any event, they are fundamentally changed by their experiences.
Tied to this is my disagreement that "The Colour Out of Space" has little of plot. In the broadest sense, this may be true, and would certainly fit your description; but plot doesn't have to be in broader strokes, it can be in very small touches as well (for example, A rebours, the bulk of Hawthorne's The Marble Faun, several of Henry James' short works, weird of otherwise). I would say that is the case here. There is actually plenty going on, as incident piles on incident to indicate the true state of affairs, and the effects on the various characters are quite different: the Gardners, worn down mentally, physically, and emotionally by the influence of the thing, become numbed to it, accustomed to stranger and stranger things, including the disappearance of other members of the family as well as the altered taste in food, etc.; Ammi becomes more and more reluctant to continue his friendship with them, and begins to withdraw; the narrator, even second-hand, comes to feel more and more the presence of something which violates all his previous conceptions of reality, going from a fairly solid emotional state to one who is filled with fears and night terrors and visions of an ever-spreading blight.
And, despite HPL's own stated lack of interest in characters per se, he also stated that the human characters should be presented with "unsparing realism, not catchpenny romanticism", in order to make the acceptance of the phenomena which lies at the heart of the tale more reasonable. Hence, as said, his protagonists seldom make such leaps as most reasonable people of these sorts of backgrounds would be unlikely to make. On the other hand, often his handling of characterization is quite subtle, as with Lavinia Whateley, who is given a distinct air of pathos in a very understated fashion; rather than making her melodramatic and the center of attention, she is presented as a pathetic creature who dies an unknown and unlamented death, with the implication that she faces it alone (again, Lovecraft isolating his character for maximum effect).
I don't really agree with your statement about his plots not standing up to scrutiny, generally speaking, though I do think there are certainly instances of such; and I would tie this to your later statement about the protagonist overlooking obvious implications... then again, as noted above, in the real world, people with the sorts of backgrounds his protagonists have would usually be blind to such simply because they are such violations of the accepted "natural order"; they are more likely to doubt their own perceptions or intuitions than to accept the implications which are, frankly, so beyond a naturalistic view of the universe. As with his protagonists, it would take an extraordinary amount of such to overcome their scepticism about the preternatural. So, while it may seem the cliché -- who hasn't felt frustrated with the protagonist of a horror film or the like who just seemed too darned thick at times -- I would argue it is actually a very reasonable and accurate presentation of such characters when faced with something which is so at odds with their fundamental perceptions of reality. "Core beliefs" override evidence time and again in everyday life, and we are all prone to this sort of blindness in one area or another.
Incidentally -- I would argue that there are severe plot holes in The Turn of the Screw, even though I think it a marvelous performance. There have been various essays over the years pointing this out; I don't have a list, but if you look about a bit, I would imagine you'd find them fairly easily.
Wilum: I think that yes, his plots did make his work original; certainly for his time. Even Neil Gaiman, who criticizes "The Call of Cthulhu", also admits its originality of conception. Not all of his works are that original, of course; several of his earlier stories were much more closely allied to older models of the weird tale; but as time went on, he did indeed create his own type of tale, working out particular methods for telling such. As for the science fiction magazines and the like influencing him... perhaps, to some degree, though I think that was relatively little. After all, he largely disdained what was being published there as simply crap (with a few notable exceptions), and didn't bother reading most of it through, he found it so hackneyed and predictable. But what he saw as the best of it may indeed have given him some ideas about how to handle certain aspects of his own tales. However, by the time he was attempting to publish in these, or saw publication in them, his fiction writing days were practically over, so the influence was not likely to have been that great. I could be quite mistaken, of course, and someone else may have some very good indications that I am; but I would say that this was a very minimal thing in his life, and nearly all the aspects which might bear some comparison with the approach of the science fiction magazine stories were there from early on.
Dale: On the subject of "The Shadow Over Innsmouth"... yes, I believe he did. I may not be able to reply in detail to any questions on that right away (once the work week starts, time becomes very scarce, I'm afraid; even with the change in management, that hasn't changed), so the discussion may be somewhat attenuated -- but if no one has any objections to such, I'm willing to undertake to make a case for such a position.