No. Demonstably means it can be demonstrated. So you can show me some fake gold and tell me about it, but to demonstrate you were conned, I’d need some more evidence. After all, I have some fool’s gold, but is was sold to me as such. I wasn’t conned. I can sort of demonstrate I wasn’t conned because there’s a label on it saying what it is, but I whether I was conned or not is likely impossible to demonstrate conclusively because being conned is about the communication and understanding, so there’s rarely a lot of evidence unless there was a recording of the conversations. It’s even harder to prove the absence of something, so for your second question – it’s almost impossible demonstrate no one else was conned unless you specify what context (certain people, during a certain time period) and then provide some evidence that it didn’t happen (like you have evidence they were locked in a cell or didn’t have physical access to the other potential victims, or something).