If you had X theme installed and enabled on the site prior to activating Pro then that must have been the reason why Cornerstone was installed on the site as there is no other way for Cornerstone to be installed on a site except when someone installed it or X is previously active on the site since that will automatically install Cornerstone too.
The only thing you have to make sure before Pro is activated is that Cornerstone is either deactivated or deleted on the site which I believe was already explained in the previous response on this thread.
So if you still have Cornerstone active on the site, you can safely delete it through FTP.
You should be able to this button so that you can edit page using the Pro Builder:
If you are not seeing either of these, please check the Permissions Manager and make sure that the Pages is enabled.
You can certainly use Cornerstone to build your posts. You just have to make sure that you enable the Posts in the Permissions Manager so that you can edit posts using Cornerstone.
Hope this helps.