Meeting dishonest ex-supervisors at large international conference -- Should one speak out?

There is a lot going on here. If you want to play power politics make sure that you have the power first.

You contemplate making an accusation. Depending on local law and custom this could open you to a charge of slander that you would have to defend. Far in the past a duel on the green would be seen as the appropriate response. Now it is lawyers instead of pistols, of course.

If you can make the claim fairly and can also do so anonymously you protect yourself. If the journalists you intend to speak with can really protect "their sources" (i.e. your identity) it might be possible. But they will need evidence even though they are less susceptible to a charge of slander.

But an emotional response won't help anyone and might reflect badly on yourself.

But I see your dilemma. Others may be at risk here if you don't make some attempt. I think that the conference chairs that I have worked with would listen to your complaint if you can contact them. This won't guarantee anonymity, of course, but in the best case it could put other powerful people in your camp. On the other hand, if the committee is somehow aligned with the PIs in question it would be the worst situation of all.

One rather scary possibility is that you confront them privately and tell them your complaints and that you don't like to see them recruiting others into the same situation you faced. If you know of any others who already faced the same situation and you can confront the PI as a group it would be even better. Some people have no shame at all and some are driven by forces even they can't control, but you might let them think that withdrawal is a better option than continuing. Scary, though.

Ethics don't require, in a case like this, that you put yourself at risk to save others.

If I might propose an alternative?

Your goal here isn't necessarily to accuse the supervisors in question, but to warn potential vulnerable victims to their scam. I don't work in academia, but I do sometimes have to cover the topic of government corruption, and avoiding slander can sometimes be simply the case of not accusing anybody directly.

So instead of saying these X people did Y terrible thing to me, consider:

I experienced Y terrible thing that has A, B, C traits (EG Chinese document that contains a dodgy contract, being paid less than stated amount).

So in your case, what you're doing is giving the people sufficient information to identify the scam/trap themselves (without saying who is responsible. If anyone asks, you can simply decline to name). Anyone reading between the lines will see what X person(s) are doing fits A, B, C traits and put two and two together. If X person(s) accuse you of accusing them, you can simply point out you never even mentioned them by name.

For them to argue it's slander, they would have to prove that their actions matches the ABC traits, which would mean they would effectively be proving it is a scam.

For handling them directly, I would propose you alert the appropriate legal/goverment/police department that handles fraud (potentially as an anonymous tip-off, but more useful if you were a named source), because if they induced you into accepting a contract via deception, what they are committing in most countries is effectively a crime, and it's likely you're not the only person they've scammed, and they appear to be, as you say, hunting for more.

It's worth adding: If they've left you out of pocket, you might even be able to get a no-win-no-fee lawyer to chase the costs back. Especially if you still have all the documentation to prove what you got wasn't what they induced you to believe.

Summary:

Your goal here is to use what you've learned in order to alert people to the signs of a possible scam so they don't fall for the trap. It might not just be the people you've met that are engaged in such practices, so it's probably an excellent idea to make it a warning in general.

Any matters of dubious legal practices should be handled legally and not in the public arena.

I advise you to talk with a lawyer ASAP.

If it were in my country the embassy/consulate was involved, as per work law, I can choose the jurisdiction as my home town...and so, being the jurisdiction local, local laws kick in, and a signature of mine in the Chinese version of contracts would be mostly nil because I do not understand Chinese.

However there might be lengths of time to do that, 1 year at most year in my case, so hurry up.

Some of the charges besides monetary might be criminal...as said, pay a consultation with a lawyer to clear out if you have any leg to stand on in a case.

I would also talk with your University body to see it is worth filling a written complaint that prevents them to deal as an accredited academic reputable entity locally. If you can find others that were also wronged, so much the better.