Ionic 1 uses Angular 1 and Ionic 2 is based on Angular 2. As Angular 1 and 2 are very different versions (almost like different frameworks) - there is no option no compatibility option.
Is your question whether you can migrate an Ionic 1 app to Ionic 2 app?
For now yes. Although there is now Ionic 3 beta so we might jump to that version. Too early to confirm.
Please read more here: https://docs.appery.io/blog/upcoming-...
NFC support can be added with a 3rd party Cordova plug-in.
Yes, as an API is invoked from the server, cross-domain issue won't be a problem.