Even if this kind of extortion was done and sanctioned by law enforcement the evidence of payment would mean nothing and those doing the extortion would be guilty of extortion. Also any other evidence collected by these agents would probably be inadmissible.
Just because you are doing an investigation does not allow you to extort a suspected criminal. Telling others about an extortion attempt in advance doesn't remove criminal liability, it only potentially adds a potential conspiracy charge.
Whether evidence becomes inadmissible it depends on which country you are talking about. In this case the relevant country seems to be Austria. I am not sure whether many people know how Austrian laws work. Just because something is true in US law it does not mean that law is relevant anywhere else.
I am pretty sure in my country no evidence ever becomes inadmissible. Everything goes. Execpt if there were laws broken when collecting the evidence there could be some repercussions to the one breaking the law but whatever information is out there, it does not disappear like in US law it does.
However that being said I do not understand at all the extortion plan. There is very huge downside for both person doing this one but also for the whole community as well and very little upside.
There are many people who do not understand at all how bitcointalk seems to allow all kinds of nefarious stuff going on on their forums and now we can reasonably think it is because people in authority positions in bitcointalk are running an extortion ring and getting their cut from the stuff that goes on in here.
Also I don't understand the timestamping of the message. I seems like just smoke and mirrors. Timestamping it like that does not prove that the encryption was not done on some later hour like the only sensible thing you might want to prove in this kind of instance would be. And even that would fall far short from proving that the extortion ring did not really mean to pocket the money by keeping everything out from public.
Only some kind of irreversibly forced publishing of agreements provably written before the extortion would prove that the ring did not really plan to keep everything secret and pocket the money and have the coming clean only as plan B.
