Well, not quite, it's a bit simpler than that. Assigned Users lists the users who currently have a CAL--those are the only ones you will probably care about, as this list should show you leased users as well (not just AccessPoint). The License Lease History is useful for seeing who has leased in the past.
When you have 5 users under Assigned Users, that means you're out of CALs. 5a: that's true, but you can force this to happen earlier by deleting all PGO files (although this will clear all users, not just quarantined ones). You need to delete the users manually after the quarantine period, although a restart of the QVS after the period will delete the user as well. Note that only those users that have been active in the past 24 hours will be quarantined. If a user has not been active, you can remove him immediately.
6a is an interesting question: yes, the user will probably keep his leased license because it doesn't do checks once the license is successfully leased, but that might be a license violation if you've removed the user. To get rid of the leased license, you need to delete certain registry settings and/or files, depending on the version of QV you're running. I'm not sure I understand 6b, but a new user can "come in" as soon as you have at least 1 CAL that's not assigned.
Btw, if you really want to assign users manually, you might want to consider unchecking Allow Dynamic CAL Assignment...
A much belated THANKS for your answer - for some reason I did not get notified about your post. Because I am an occasional user/admin I did not check back until I saw your reply in the QlikCommunity Updates. But my questions remain and your detailed/expert answers are very helpful - I hope for others as well. Hope version 10 has made improvement on the CAL management.