Is it possible to buy, install and use Windows 10 without ever being online again after the software has been checked for authenticity by Microsoft (as is done now for Windows 7)?
- yes as with any Windows , it works offline (except for those Apps that need the Web)
- You can even update offline and more http://www.portableupdate.com/
Short Answer: Yes, you can.
There is even an off-line authentication for your license key, so potentially you may never even 'need' to get on-line, however once you do establish a connection to the internet, whether it is via Wi-Fi or ethernet, Windows 10 will attempt to authenticate and perhaps update as well!