420, how can you prove that your mandarin was caught OR exposed to cyanide, and not the
much more likely fact that he probably starved in your tank? Have you ever seen just how large and fat mandarins can really get in the right setting? Are you aware that it's not just the
amount of live rock in a system, but the
quality of the live rock as well? Did you know that it usually takes many, many months (even up to a couple of years - assuming it's got "something" to work with) for the full complement of pod species to reinhabit rock that's been shipped? Yes, the "long term" effects on fish to cyanide exposure are going to look a whole lot like what happens to an animal that's starving, but one does not necessarily equate or negate the other, and it's generally accepted that mandarins most often die for lack of suitable diet.
Don't just take
my word for it, talk to some folks with vastly more expertise on the subject, how about Anthony Calfo, Bob Fenner? Try a search for his opinion (or you can try querying either of them directly through wetwebmedia) on the subject (but if you mention me, you'll get put in my inbox

).
As for collection with cyanide, what proof more do you want, a talk with collectors? It's well known in the trade that these animals are not caught with cyanide, haven't y'all ever noticed the perfectly round little holes that they come in with? Haven't you ever wondered about that?
Also, brine, live or otherwise, are an entirely unnatural and unsuitable primary foodstuff for mandarins, and they cannot be expected to live full spans nor to reach full sizes on such a diet. If they're eating frozen or otherwise, such as the Zooplex, and they're thriving long term, then this shouldn't be discounted.