Thank you for your follow-up. Please bear with me as you have a very detail oriented response.
First of all I am unaware of any specific treaty but what you can do is retain local counsel in whatever state you decide to pursue your claim, go to their local courthouse, and pursue 'domestication' of judgment. It is essentially a request that the local courts treat the judgment as if it were granted by the existing court, and consequently then permit you to collect on the judgment based on that state's and that jurisdiction's rules. For example if you choose to domesticate in California, you would then be able to pursue collection of this judgment under California law and California restrictions.
In terms of limitations for defamation, I will go state by state and give you a quick rundown of statutory limitations as each state had different laws. You would need to pursue each party individually or find one location where you can file a massive suit in federal court of that state against all of the participants. Under US law if you file in a Federal court, those courts follow Federal guidelines but would still be bound by state statutory limitations and local precedent.
The statute of limitations for defamation in Michigan is one (1) year. Mich. Comp. Law §(NNN) NNN-NNNN7) (1961).
The statute of limitations for defamation in Washington is two (2) years. See Wash. Rev. Code sec. 4.16.100.
The statute of limitations for defamation is one (1) year in Arizona. See A.R.S. § 12‑541.
California's statute of limitations for defamation is one (1) year. See California Code of Civil Procedure 340(c).
The statute of limitations for defamation in Illinois is one (1) year. See 735 ILCS 5/13-201
For Minnesota this is two years from the date of publication. Minn. Stat. 541.07
Missouri has a two year statute from date of publication. http://law.justia.com/codes/missouri/2005/t35/5160000140.html
The statue of limitations for defamation in North Carolina is one (1) year. See N.C. Gen. Stat. § 1-54.
Oklahoma has a 1 year statute of limitations and so does Oregon.
In the US defamation claims are very uncertain. It is very unlikely that you would be able to find an attorney to take this on contingency, especially based on the complexity and the amount of parties in this matter.