1.  The Mann Act involves travel over state lines with a minor for immoral purposes.  Don't bring any minors with you and you'll be fine.

2.  To the extent that any activity (e.g.phone calls or emails) took place in Massachusetts, then techincally, yes, you probably violated Mass laws, but in practice, the likelihood that a Mass cop will find out and travel to RI to bust you is next to nil, or less.  Certainly no Mann Act violation.

3.  If you have no contact with the underage girl, then you should be in the clear; but the owners of the AMP sure will be in hot water.  At worst, you could be called as a witness if you saw the person there; but there's not a high liklihood of that.

4.  I'm not sure about any recent changes per se, but there has been numerous discussions of the fact that RI does not prosecute for activities that take place behind closed doors, so the State has become something of a Mecca for MP's lately.  I say more power to them but remember, the pendulum swings both ways.

(still not a lawyer)

refer you back to the answers provided on the Boston Board where you posted back then.

First, get a lawyer and pay him. Free advice is worth exactly what you paid for it. If the stakes are too high, then isn't a lawyer worth it?

Second, it might be helpful to read the statutes in question. I would read Title 11, Chapter 34 of the General Laws of Rhode Island, which are the prostitution and pandering laws.  http://www.rilin.state.ri.us/Statutes/TITLE11/11-34/INDEX.HTM

If you have questions about the Mann Act, also known as the White Slavery Trade Act, read 18 USC 2421 at http://www.access.gpo.gov/uscode/title18/parti_chapter117_.html:

Whoever knowingly transports any individual in interstate or foreign commerce, or in any Territory or Possession of the United States, with intent that such individual engage in prostitution, or in any sexual activity for which any person can be charged with a criminal offense, or attempts to do so, shall be fined under this title or imprisoned not more than 10 years, or both.

There is no age requirement in 18 USC 2421. 18 USC 2423 does make it a crime for transporting minors for the purpose of prostitution. The key word in this section is "transport another". You going across states lines to find a prostitute is not a violation of the Mann Act. You somehow inducing a prostitute to cross state lines is violating the Mann Act. You hiring a woman from another state to make a pornographic movie violates the Mann Act. Pecuinary gain is not necessary for prosecution under the Mann Act.

The part of the Travel Act concerning prostitution is at 18 USC 1952. http://www.access.gpo.gov/uscode/title18/parti_chapter95_.html

(a) Whoever travels in interstate or foreign commerce or uses the mail or any facility in interstate or foreign commerce, with intent to--
(1) distribute the proceeds of any unlawful activity; or
(2) commit any crime of violence to further any unlawful activity; or
(3) otherwise promote, manage, establish, carry on, or facilitate the promotion, management, establishment, or carrying on, of any unlawful activity, and thereafter performs or attempts to perform--
(A) an act described in paragraph (1) or (3) shall be fined under this title, imprisoned not more than 5 years, or both; or
(B) an act described in paragraph (2) shall be fined under this title, imprisoned for not more than 20 years, or both, and if death results shall be imprisoned for any term of years or for life.
(b) As used in this section (i) "unlawful activity" means (1) any business enterprise involving gambling, liquor on which the Federal excise tax has not been paid, narcotics or controlled substances (as defined in section 102(6) of the Controlled Substances Act [21 USCS § 802(b)]), or prostitution offenses in violation of the laws of the State in which they are committed or of the United States.

Though the Mann Act is more famous, the Travel Act is actually more expansive. The Travel Act has been used to prosecute both prostitutes and their customers. For example, under the Travel Act, paying for prostitution with a credit card is enough of an interstate activity to invite prosecution, since credit cards are part of interstate commerce. United States v. Walton (1986) 633 F.Supp 1353. You going across state lines to visit a prostitute violates the Travel Act. You hiring a prostitute to cross states lines to come to you violates both the Travel Act and Mann Act, though you can only be punished for one. The big catch to the Travel Act is that it has to be a violation of state laws. Also, it has to be a business enterprise, but it's a low threshold to be a business.

These are federal laws. Prosecution under federal laws does not bar prosecution under state laws unless the state forbids it. A federal and a state prosecution does not subject you to double jeopardy.

This post isn't meant to say what will or will not happen. It's only meant to explain current federal law.