Yes, you can use any license version with any program version. If you have an older license then some new features may either be grayed out, or a message may be displayed indicating which license version is required to use that feature.
Yes. When a license is purchased for two or more users, then there are standard pricing discounts.
AYS licenses are based on the number of users that may simultaneously connect to an individual database.
You may install AYS on as many different computers as you wish. AYS licensing limits only the number of users that may simultaneously connect to an individual database, not the number of physical computers on which the software may be installed.