It is not free software but it is open source. Stop gatekeeping the term. I can look at the code and modify it to my hearts content. I can also watch as the project is being developed. That means it’s open source. It would be free software if you where also allowed to redistribute it but I can fully see why they do not want that
Use tauri instead. Uses less ram and disk space
No. You can only return a single type from the function. You could return the serde_json::Value though so that the code calling this function can get the value it needs itself