难道：1 adv. used to reiterate a rhetorical question 河水难道会倒流吗？rivers never flow backward, don't they?
NOTE: the word 难道 can be echoed at the end of a sentence with the rhetorical word 不成 bu4 cheng2 难道他病了不成？ He isn't ill, is he?
莫非：the same meaning as 难道， also can be echoed with the word 不成
indicating guessing or questioning
今天她又没来，莫非又生病了不成？She is absent today, can she be ill again?
万一：in case, eventually, if by any chance
1 good and bad, what's good and what's bad
2 mishap, disaster , risk
万一她有个好歹可怎么办？what if something should happen to her?
3 no matter in what way, anyhow , make do with sth 无论条件好坏，将就的做某事
时间太紧了，好歹多吃点就行了。the time is pressing, we'll order whatever there is.
4 in any case, anyhow, at any rate 不论如何
她要是在这里，好歹也能拿个主意。If she were here, she would give us some advice at any rate.