To 1)
I have not implemented a method to check if the server itself is running MSAC yet as there is no way to do so for me. Adding a function to check this would break the scripts if they are running on non-MSAC servers since the function will not be defined there.
To check if MSAC it running, call GetPlayerStat(1, 'MSAC.GUID'). If it equals an empty or nonempty string, MSAC is running. If the type is not a string, MSAC is not running.
To 2)
You are right that GetPlayerStat(ID, 'MSAC.ClientInstalled') isn't anything but an alias to several MSAC.Status queries. However, in case that I ever add new status constants, MSAC.ClientInstalled will still return the correct value.