I see you pretty much got the question answered. Just a few more things that hopefully would be helpful.https://generator.webtrends.com
is where you can generate the REST API URL. You use your the standard login details. When importing the data in Excel it is good to know that in the user name field you actually need to fill: account\user.
Webtrends Explore may be a good option to get the data but there isn't an easy way to get the data by month. And the login for Explore is not the same as your standard login details. You may need to ask your client to give you access to Explore. Generally, it works fine. It is something like GA custom reports but... lovely.