See https://www.rhino3d.com/6/new/licensing-and-administration for details.
Yes, using either the Zoo or the Cloud Zoo.
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.
Add your licenses to a team in the Cloud Zoo (recommended), or add your licenses to the Classic Zoo.
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 6.1 to support Rhino 6 license keys. The Zoo will group your Rhino 5 and Rhino 6 Upgrade keys into one license, 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.