weekly interest only comes from new investment.
Unless you can prove this, you should not use such statements.
He can, and he did. No other conceivable substantial source is known and this has been true of every similar claim in the past. You don't have to rule out every inconceivable alternative to have proven something. Proof doesn't require omniscience, just reasonable certainty.
In context, it's clear that "only" doesn't rule out the possibility that some miniscule amount occasionally comes from some other source. "Only" can mean the only substantial factor or effect. For example, "Don't argue, you'll only make him mad" doesn't mean that you won't also cause air to vibrate.
Although I appreciate the fact that you are trying to warn people, you should say: 'I think this is a ponzi' or 'I strongly believe this is a ponzi' or 'If this is not a ponzi, I will give you all my money*', but not 'This is a ponzi'
There is no difference. When a person makes a claim, they are stating that they believe that claim is true. We understand that people are neither perfect nor omniscient.
If I say "Your wife is cheating on you", every rational person who understands the English language knows that I mean that I have reason to believe their wife is cheating on them. You would not conclude that it must be the case that she is in fact cheating on you. Nor would you believe that I was stating that logical proposition just to bring it up for discussion. The most natural interpretation of "Your wife is cheating on you" is that the speaker has reasons that he believes justify believing that your wife is cheating on you. "I think your wife is cheating on you" says precisely the same thing, just more weakly.
There is absolutely no reason he should weaken his warning. In fact, if he could think of any way to make it stronger, he should do that.