Hi there,
Unfortunately, the question is so confusing and as we do not have any context we don’t know exactly what is going on. You will need to give us each question in a separate thread that we can focus on each issue, also it is indeed counter-intuitive to answer the question here as t is not related to the original post. Kindly open up a separate thread for each part of the questions (Validation and the error 500) and we will be more than glad to try to help.
Also please add the Login information to each thread.
Now to answer the questions briefly and having the follow up for the separate threads, you will need to enable the WordPress debug mode to see the actual error instead of 500 and give us the error message so that we can follow up. Here is how to enable the debug mode:
https://codex.wordpress.org/Debugging_in_WordPress
I checked the validator and it seems that there are shortcodes in the page that are not rendering, is the Cornerstone plugin activated while you were checking the code?
Please add a spearate thread to follow up.
Thank you.