- Oh, I think Heathcliff is the kind of guy who would definitely get a kick out of raping someone he despised, especially if he could get away with it (which he could, and did) and benefit from it (forcing said person to conceive an heir/tool to his ploy).
- It's been many years since I read the book, but I thought the implication was that Linton looked more like his mother. If he did, it would be hard to tell if Heathcliff was his father, but ultimately doesn't really prove anything either way, unfortunately. It's probably left ambiguous on purpose.
- Linton was only born a few months after Isabella left Heathcliff, though. Unless by a few Nelly actually means nine or ten, there would have been no time for Isabella to conceive him with a man other than Heathcliff.
- From his description in the book, I always thought Heathcliff was meant to be Roma. People in Europe didn't always look favorably on them, to the point of completely distrusting them, and they are associated with fortune-telling and magic. They had a reputation among some as being "noble savages", kind of like how European settlers regarded Native American Indians. Roma were also assumed to be thieves, which could explain how Heathcliff accumulated his wealth. Bronte does refer to him as a gipsy, and she might have meant it literally. On the other hand, he is described as though he were Raised by Wolves more than by humans.
In keeping with the other guesses that he's some sort of supernatural being, Heathcliff is one of The Fair Folk, who Mr. Earnshaw was tricked or beguiled into taking home with him. This explains his Lack of Empathy, "wild" nature and preference for the natural world over civilization, and why no one can initially figure out what language he speaks. He's also a trickster, swindling Hindley out of his money and property; do not gamble with the Fey, people. And despite his thoroughly repellent personality and behavior, he has an undeniable charisma and can charm those around him when he considers doing so worth the effort. Also, visiting vengeance not only on those that wronged you but their descendants as well is a very faerie thing to do, as is claiming their children as your property.