You raise some very interesting points, and I must thank you both for the discussion in general and for that story about Palmer in particular; this was something I was totally unaware of.
On the subject of Hawthorne: I have by no means read everything of the man's, but I do have a huge selection of it set aside for reading (and, in many cases re-reading) following my (extended) reading of Poe (which has included not only his fiction and verse but letters, critical essays, reviews, and any other documents by him I have been able to come across; for the tales, sketches, and verse, where possible I have been using Prof. T. O. Mabbott's annotated edition). I have read large portions of Hawthorne's notebooks before, though, as well as most of his novels and his complete short stories, and some of his letters; but I am very much looking forward to doing so again, as Hawthorne has always had a firm place in my estimation.
On Mother Radcliffe: I hope you enjoy Udolpho, though that is by no means the easiest of her books to begin with (I am assuming it is the first; though I could be misreading you there). I read them in order, beginning with The Castles of Athlin and Dunbayne, and have read all but Gaston de Blondeville several times now, my appreciation for them growing each time. That one exception is due to two things: I do not have a copy in my personal library, unlike the others; and I found that, though this is the one instance in which she kept the supernatural as a real phenomenon, I felt that the writing here was much weaker overall. It seemed to lack her usual polish (or finish, if you prefer), making it feel at times almost makeshift in comparison to her other works.
As for your points about "The Picture in the House"... I can see where you are coming from; it just doesn't tend to strike me that way. Incidentally, the narrator doesn't "hightail it out of there", but freezes, simply closing his eyes, only to open them again on a scene of devastation following the "thunderbolt". A point which has always stirred some controversy, as it leaves it open to question how the narrator survived, whether what he encountered was a "ghost" house and its inhabitant, whether the stroke of lightning was a judgment of some sort (something which would be completely alien to Lovecraft's usual atheistic approach), and, indeed, just how reliable this narrator actually is. There have been some interesting articles written on this one over the years, as a result....
No, I don't think Lovecraft's longer pieces necessarily "need" any such humorous interlude. I tend to agree with him that, in the main, when it comes to works centering on terror, such actually dilutes the effect, often making it nearly impossible to regain the tension and genuine atmosphere of the otherworldly. However, this is not to say that Lovecraft doesn't have some lighter touches. There is some dry humor in "The Whisperer in Darkness", I think; and certainly his comment about Gilman studying too hard in "The Dreams in the Witch House" would seem to be an excellent example of such wit, as it serves as a rather acidic comment yet increases the horror of the situation. Ward does, I think, have one moment which may or may not have been intentionally of a humorous nature. That is during Charles' defense of his incantations and the sort of psychological realm he must inhabit in order to get full benefit from them. This smacks of exactly the sort of occultism Lovecraft derided when used by others, and has always struck me as an awkward passage as a result. He may, indeed, have intended this as a slap at exactly this sort of nonsense in weird writing; but, if so, I fear that it still constitutes, if not an artistic blunder, at least an artistic misstep. And, of course, The Dream-Quest of Unknown Kadath is peppered with bits of humor, both dry and otherwise (I believe it was George Wetzel who remarked on the "gargoyle"/"gar-ghoul" pun concerning Carter's encounter with some of Pickman's cohorts).
