What is an easy and accurate way to get active Scale users?

I'm not aware of any report that can provide this information.  You could try the options you suggest but it's not going to be accurate.  I guess a way to do this in the future would be to use the security feature in ZS - at least that would give you some upper limits of users, but it still wouldn't tell you about actual usage and activity.

I would try SmartBear support and see what they suggest - perhaps they can run a report over your system to give you that information?

SmartBear Support