In 3.6.x there is now a path for php-cli which probably isn't set. Search Admin interface for path. The path for php-cli suggestion you see in that config page is not an autodiscover ... it's a suggestion.
CentOS 7 most likely is /usr/bin/php but double check from an ssh session into server with /usr/bin/php -v
Output should have PHP version number (cli) on the first line.
Then check the running of cron via command line:
to see that something is hanging.
Things check out then path to php-cli is /usr/bin/php
Tis 'spring cleaning time' at your Uni? Someone getting ahead of the time to cleanup? Are resources in courses or courses in categories being removed?
Recyclebin's for resources in courses and for courses in categories might have been the culprit in increase of disk usage. File names you see are hints for checking for recyclebin links in courses and in categories.
Query of DB something like: mysql> select component from mdl_files where component like '%recycle%';
There is also a new script for adhoc_task in moodlecode/admin/tool/task/cli/ called adhoc_task.php that you also might want to check.
Soooooo ... ????
'spirit of sharing', Ken