I would like to know if it is possible to let another iOS app open a file my app has created and then the other app edits and saves it just in the original location where my app had saved it at creation time. Or, instead, are files opened with the NSUrl in UIApplicationLaunchOptionsURLKey just read-only so they just can be saved in the other app sandbox?
You can't edit other apps sandbox, it's private to your own app (except for the Documents folder, that a user can access from iTunes).
You also don't have access to the file system of the phone.
Welcome to iOS :)