[susan53]

a) It would be more usual nowadays to use full stops rather than semi-colons. The use of semi-colons to divide main clauses, though still seen, seems to be dying out and often has an "old-fashioned" feel to it.
b) These are all concession markers in a clear context of concession. As I said above, concession conjunctions and adverbials express a change in the direction of the discourse - something unexpected in the second part given the logical conclusion you might draw from the first part. There is therefore always an element of "surprise" with concession. The degree has nothing to do with the connective but with the content of the text.
If you look again at my last post I said that yet was more "surprising than "but" in your example because with "but" it was unclear whether the speaker intended simple contrast or concession - from the content it could have been either. With "yet" there is no ambiguity as "yet" can only express concession, not contrast. By choosing a clear concession marker, the speaker is telling you that you should be surprised.
In my example, on the other hand, as the content is clearly concessive and there is no ambiguity, the listener automatically understands "but" as indicating concession, so that there is no difference in meaning between any of the sentences.