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 does not. Rhino 6 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 6 features - you are not adding a new license. This means that you can use Rhino 6 and older versions of Rhino on the same computer. You may not use Rhino 5 on one computer while someone else uses the Rhino 6 upgrade on another.
Yes, Rhino 5 can be installed on one computer, even if it has been upgraded to Rhino 6.
No. Rhino 5 does not support the Cloud Zoo.
You'll need Zoo 6 SR10 (or higher) to support Rhino 6 license keys. The Zoo will group your Rhino 5 and Rhino 6 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 5 and Rhino 6 from the same computer will use only one license.