Hello There,
Thanks for writing in! Since you are way behind, I would recommend that you create a staging site. This is very useful because you can test drive any X updates in the staging area, do the customizations and troubleshooting without disrupting your live site. Once everything is perfect, you can easily and flawlessly proceed the update in the live site. To know how to create a staging area, please check out these articles:
http://www.wpbeginner.com/wp-tutorials/how-to-create-staging-environment-for-a-wordpress-site/, https://maintainn.com/2015/02/how-to-create-a-staging-site-for-wordpress/
In your staging site, you can update X theme right away. Please also update Visual Composer. Once updated, you should be able to edit the page again in Visual Composer. Please do not worry about Cornerstone because page created with VC will still work as long as VC is active.
Please deactivate and delete X Shortcodes 3.0.5 plugin. Please take note that X shortcodes 3.0.5 and its functionality has been consolidated into Cornerstone which will be installed automatically if you go to X > Overview (assuming you already have X5.1.1).
Now if you want to create a new page and would still want to use Visual Composer, you may do so. If you want to create the new page using Cornerstone then you should be able to do that too.
Please let us know how it goes.