I a. and b. are both correct, I think, but they are confusing. They could probably be improved by changing the order of the phrases, and or adding additional punctuation. Ex: If he happens to be something else (which he must be these days to live), even a poet may carry a briefcase. In 2. I'm pretty sure it should be THAT instead of WHICH, but it's impossible to say for sure, as we don't know the original intent. Google 'that vs which' and you'll get the idea.

To me it feels like both under 1 could be correct. Their meanings are different. 1a feels like the poet in question is a random poet (any poet). So, the 'he' in the if-clause refers to any poet as well. In '1b' it feels like the 'he' is a specific person and 'poet' is not a person but an occupation. It seems that this 'he' is changing jobs and even if he becomes a poet and on top of that does something else, too, he may need the case. 2. The relative pronoun 'which' may refer to 'best' - You could say something like 'The men that the country village and the urban housing estate need must be the verb best which can be found.' 'Which' may replace qualifiers 'Which turkey? - The big turkey.' Them = the country village and the urban housing estate both - both of them