Can’t access dashboard “All Pages”

Files from the WordPress core may be missing here. I would recommend that you carry out a manual update: https://wordpress.org/documentation/article/updating-wordpress/#manual-update

Yes, when updating manually you should replace all files as described.

However, your further description sounds questionable. I would first recommend that you read this article here: https://wordpress.org/documentation/article/faq-my-site-was-hacked/

Then you should run a malware scan, either with the security plugin you are already using or with WordFence: https://wordpress.org/plugins/wordfence/

If anything is found, I would recommend deleting everything – all files, directories and also clearing the database. Then you can restore the project from a clean backup. If you don’t have a backup, ask your hoster’s support.