Re: It would have been fun.
1. 'It will have been fun' expresses certainty about the past. 'It would have been fun' is less certain. Additional context might also reveal that the speaker is actually suggesting regret that s/he did not go - (It would have been fun if I had been able to go).
2. This is indeed counterfactual. The speaker did not go to the concert and did not see Taeyoun.
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