岂, 莫非, 难不成 — rhetorical questions, raised

规则 岂 = how could — CLASSICAL, a question that already contains its DENIAL: 岂有此理！ 岂能如此？
岂不是 = is that not…: 这样岂不是更好？
莫非 = could it be that — a STARTLED GUESS, often closing with 吧 or 不成: 莫非他忘了？
难不成 is the spoken 莫非: 难不成他不来了？
难道 is the neutral member of the family and fits any register.