A speculation, is it likely Heathcliff raped Isabella?

I would say yes and no. His whole goal with marrying Isabella was to have a child with her so that the child would inherit Thrushcross Grange. He probably went about having sex with her in a "business-like" way to get her pregnant as quickly as possible. My guess is that she was miserable and uncomfortable and in pain during the experience, but most likely didn't say "No" or fight him or try to refuse him. So it wouldn't exactly have been voluntary on her part, but she probably didn't tell him 'No', either.