Glad you sorted things out and thank you for purchasing Cornerstone license. But it seems you got confused, base on your screenshot you were not able to install the X theme or maybe you did but it's not complete, why I said that is because I don't see the X panel between the Dashboard and Posts panel (see your screenshot above).
But somehow you were able to install the Cornerstone, and because you did not have the X theme the Cornerstone acts as a Standalone, that is why it asks for validation, but you were not able to use your first validation code (the one you showed above) because that is for X.
If you were able to install the X theme and validate it, it will automatically install the bundled Cornerstone and will not ask for validation.
Besides from ThemeForest account page, you can also download an updated X theme from the APEX Dashboard.
I think that is part of the issue, because on your screenshot it seems you're uploading the package zip from ThemeForest, but you should have extracted that first and upload only the
x.zip file which is inside of that package.
Hope this shed some lights,