Unfortunately, I have had to edit your question down to just one question according to enotes regulations. It is after the death and internment of his sister's corpse, Madeline, that Roderick begins to prowl throughout the house as part of the "observable change" that the narrator identifies in his countenance and health. It is clear that he is far more agitated and his health is worsening brought about by his "mental disorder".

One particular night is described when the narrator is completely unable to sleep and feels a grim premonition and a mood of despair come over him. As he paces to and fro in his room, Roderick enters. Note how he is described by the narrator:

His countenance was, as usual, cadaverously wan - but, moreover, there was a species of mad hilarity in his eyes - an evidently restrained hysteria in his whole demeanour. His air appalled me...

It is then that Roderick asks the narrator, in his somewhat frenzied state, the question you are after:

"And have you not seen it?" he said abruptly, after having stared about him for some moments in silence - "you have not then seen it? - but, stay! you shall."

Having said this he moves over to the windows and throws them open to the storm. Of course, this tempest is a pathetic fallacy, in that it mirrors the internal tempest going on in Roderick and the wider storm that is occurring reflecting the curse and conflict within the House of Usher itself.