I have investigated this and found and resolve the issue, the main issue here is you seem deleted the X theme? So if we could re-install the X Theme your Cornerstone validation issue will be gone.
Now I tried to install the X manually via Themes > Upload and as you know that did not succeed because of the
The link you followed has expired. error.
Upon investigation on that error, I found that issue is caused by this:
Yes, your site's max upload file size is only 10MB, where the X zip file is 10.6MB, I installed a plugin named WP-ServerInfo to confirm that and found your server specs is pretty low (see Dashboard > WP-ServerInfo).
Nevertheless, I tried to increase the max upload file size so I can upload the X. I tried all the tricks provided here, but none works, this means your hosting/plan is not allowing you to increase these things manually.
I have no choice but to upload the x folder via FTP, and that seems to work.
Don't worry about the Slider Revolution asking for validation, since the Revolution Slider you have is a bundle with the X, you don’t need to validate it. that will function 100% without license registration.
Your concern now should be these:
Please contact your hosting support on how you can effectively increase these stuff, more importantly the Memory Limit.
Hope it helps,