They would have to violate our physical property rights to gain access to it or violate a contract.
Depends. Maybe. Maybe not. There are products that can be reversed engineered and you'd be screwed. Anything I would say about your business would be conjecture.
If he wanted to make money with his song then he should have done so. I contributed the effort and knowledge to monetize it, he did not. He lost nothing in this transaction, and I see no need for guilt.
I'm disappointed to read this.