You have some good questions. I'll take a shot at an answer.
I don't think a mixing service has an impact on your capitol gains. You are simply stating when you bought, and how much they have appreciated. You don't have to supply info on your coins. If you were audited you may be asked for more info, but if you paid it will all work out.
Some people have proposed the idea of "tainted" coins. In this scheme coins that are known to be associated with crime (and maybe mixers) become less valuable than clean coins. I seriously doubt that will happen. Almost no one is in favor of taint. It would destroy fungibility and some of the utility of BTC.
I see no specific risk other than trusting the mixing service.