2a)- I wouldn't use 'can' here, use may/could
2b & c- the first is talking about present time, and the second about past time.
4a)- This sound wrong to me
4b- possibility, but far from certain, unlike the rest, which express more certainty.
5c- correct
5d- That will have been Roland- he always phones her at this time. (I've been trying to get through to her)
