In answer to 1 - it doesn't matter how many users there are in an operating system. The licence is used by all users that can log in, but only those with Administrative authority can alter settings. If those accounts were on 2 different operating systems (as in a dual-boot situation) then 2 licences or more would be needed for them. As I interpret what you said, I think it's only using 1 licence so you're OK.
For 2 - It would have been easier if you could have uninstalled McAfee first to free up 1 licence however you can usually do the account adjustment yourself by going to your account page. If not contact Customer Service for your area HERE.
As explained above, the answer on your question No.1 is: nevermind the number of accounts, if it's the same OS the needed license is only 1.
On your 2nd question though, you need to log in in your account and see if you still have the option to deactivate licenses from there.
Some of the clients do, some do not.
If you are from the 2nd group, contact Customer Service as described above.