See https://www.rhino3d.com/6/new/licensing-and-administration for details.
Yes, using one of our free-floating license managers. The LAN Zoo works over your office LAN or the Cloud Zoo uses an occasional Internet connection to float single-use licenses between multiple computers.
You'll need a separate license key for each computer. Rhino 5 allowed more than one validation per single-user license key. Rhino 6 and Rhino 7 do not. Rhino 7 adds Cloud Zoo Personal license management so you can take turns using a single license on multiple computers with an occasional Internet connection.
Add your licenses to a team in the Cloud Zoo (recommended), or add your licenses to the LAN Zoo. https://wiki.mcneel.com/rhino/6/license
Yes. When you upgrade, you are extending your license to give you access to Rhino 7 features - you are not adding a new license. This means that you can use Rhino 7 and older versions of Rhino on the same computer. You may not use Rhino 6 on one computer while someone else uses the Rhino 7 upgrade on another.
Yes, Rhino 6 can be installed, even if it has been upgraded to Rhino 7. Make sure to have both on the Zoo or Cloud Zoo.
No. Rhino 5 does not support the Cloud Zoo.
You'll need Zoo 7 SR 7 (or higher) to support Rhino 7 license keys. The Zoo will group your Rhino 6 and Rhino 7 Upgrade keys into a license “cluster”, and will make sure only one is in use at any given time. A user who runs both Rhino 6 and Rhino 7 from the same computer will use only one license.