Tomcat8 shutdown randomly with AbstractProtocol.pause

This answer (from one of the questions you found) seems good.

Something is telling Tomcat to stop. And since it doesn't happen when Tomcat is run as root, I think the cause is some other (non-system) process (maybe a script or a cron job) sending a signal (probably, SIGTERM) to Tomcat, like kill <tomcat pid>. Maybe that other process is also run as user web - that would explain why that process can't kill root's Tomcat. Or maybe that other process just searches for processes to kill, and one of the criteria is "processes owned by web".

I suggest you carefully read crontabs for users root and web, system-wide crontab and everything in /etc/cron.*/ folders. You can also check if any other processes owned by web are suddenly terminated. And building Tomcat from source, with some tracing added (as suggested in the answer I mentioned), seems a good idea.