Look, 2 people "really like" the product.
You don't even know if it will be saleable. See, this is the thing that annoys the crap out of me. Right now, your product has zero value, because you have not started to sell it as yet. Absolutely zero.
Sell the darned thing first, see if people like it, if they do, then at least you have some stake in the marketplace and in any proceedings. You would have had the product out there, people may buy it, and you will have first mover advantage.
Right now, all you have is the possibility that someone "may" sue you, and thats just in your fevered imagination. Get coding boyo!
The moment you send out an email, it has to land in someone else's mailbox. If you're paranoid, they're plenty of other things to be paranoid about. Email is long past the point of paranoia. You have no control over what happens to the email you send, and what happened to the copy of the email that you received (in the sender's outbox).
Stop being stupid.