Can I test/develop an App without having Xcode?
It's not officially supported but you should be able to. The app delegate has a path that points to a JS server:react-native/Examples/SampleApp/iOS/AppDelegate.m.
If you point that URL to your Debian machine before building to your phone, it should work. (Of course, the JS should get bundled into the app before submission to the App Store.)
Unfortunately you can't do that for now.It supports only iOS devices and you need XCode in order to test in the simulator because following the documentation (http://facebook.github.io/react-native/docs/getting-started.html#content), it's going to create an XCode project
Unfortunately you can't.If it's because you don't have an apple computer you can always try running one on a virtual machine.