If you were absolutely clear that you gave exclusive rights to Flightsim.to then if I was you I would contact them first and discuss it with them to see what their opinion is and then take it from there.
As regards future mods could you not put any copyright info in the main program to be shown by pressing a key and locking the program down with a key combination i.e. employ some form of software protection.
Either way if you enjoy producing addons to stop now would be a bit self defeating for you and following through on legal steps can be quite costly and aggravating. If it was me, I would try and see it as an unfortunate learning experience and try and sort it in a way that doesn’t give you a load of aggro whatever that is.