Fool that I was means that you were a fool. It has no other meaning. 2a - that is the best. Though makes no sense. There is no contradiction between being a good fellow and being in high spirits. 2b - makes no sense, as explained above 2c - makes sense as there is sort of a contradiction - positive idea versus negative. But really there is no clear reason why a good fellow cannot be in low spirits, so it's not entirely clear. By the way, these structures are incredibly old-fashioned and would sound strange these days. Is this your intention?

I think Claire covered it. You want to remember that 'that he was' and 'though he was' do not have the same meaning. "Fool that he was, he slept through his exam." i.e. he was a fool; he did something foolish. "Strong though he was, he could not lift a car." i.e. in spite of being strong, he was not strong enough to do a particular thing.