Depending on which country you live in I think you have a good case with upgrading your security. I mean if it really went through the courts surely the judge would rule that since you haven't spend the bitcoin you shouldn't have to pay capital gains tax on it.

I think about this situation with coinjoined bitcoin often. If you did have to prove that you still had the UTXO, the privacy gain is now gone. Should the tax agency have to pay the fee to coin join again? I think they should, but I'm probably just dreaming.