You are still thinking that this is about the relationship between the PPT operators and the people who paid them money. It is not. It is about what they paid Pirate to do.

If I ask for donations to hire a hitman to kill my boss, I'm not ripping off my investors if I actually use the money to hire a hitman to kill my boss. The issue is what I paid the hitman to do. What did the PPT operators pay Pirate to do?

It makes no difference. Even if you think that Pirate wasn't a Ponzi, his business is still indistinguishable from a Ponzi. So this is some kind of "essence" or "omniscience" argument. ("You can't prove there's no possible way I'm right, even though all the evidence suggests that I'm wrong.") If I shoot at you with a gun I loaded yesterday, having no reason to believe anyone unloaded it, can I somehow escape responsibility because there was some chance someone unloaded the gun before I aimed it at you? Of course not. It's still wrong because I have no reason to believe the gun is unloaded and every reason to believe it's loaded. The standard for moral judgment is not a hypothetical being with perfect knowledge.

If there is no way to distinguish between the two then there is no difference with respect to the culpability of the investors for the two.

Say there are a bunch of boxes. Some are empty and some contain children. There is no way to distinguish which is which. Should you shoot at the boxes?

I hope you don't take this as rude, but you really demonstrating how Ponzi schemes and the desire for a quick, unearned buck make people throw reason to the wind.