What the son says is hearsay. As far as your reasons for borrowing the money, that doesn't matter. As the court case I cited above notes, in order to be convicted of a theft by swindle, you must have the intent to defraud another, and must act affirmatively to defraud them.
Based just on these facts, I see absolutely zero intent to defraud them. If you were defrauding them, why would you agree to sign promissary notes? Why would you make payments? Where is the intent to cheat them? Where did you purposely take steps when borrowing this money to scam them? If you had taken the money and promised that you would double it in 6 months, okay, I could see perhaps an intent to defraud. But you said you have made, and continue to make, payments. Furthermore, they signed the promissary notes. You said there were several of them. So they loaned you money and voluntarily
signed the notes. You didn't put a gun
to their head and force them.
Unless there is a lot more to this that I'm not seeing, there is not a crime here.
As for the officer, a police officers job is to find people responsible for committing crimes. They may act like they know the laws, but they often don't. They obviously don't have the same understanding as a prosecutor would. And the burden of proof for an officer to make an arrest is probable cause
reasonable belief that you are guilty. It doesn't matter whether their belief is right or wrong, which is why there are people who get arrested and then are never charged with crimes.
You have no obligation to speak to that officer, and you should not do so. Don't say a word to them, and don't put anything in writing. If they ask you to answer questions, tell them you decline and you will have a lawyer be in touch.