Does exist a method to list all jobs in crontab for all users on a system

There is no native command to do this, but you can use a simple bash oneliner like this:

for u in $(cut -f1 -d: /etc/passwd); do sudo crontab -u $u -l; done

The above would read out all user entries in /etc/passwd and list their appropriate crontab entries. sudo usage is required, since you'd need superuser privileges to access another user's cron.

cat /var/spool/cron/*

Many distributions have additional system crons configured via /etc as well. For example, CentOS has files in /etc/cron*

Let me know if you have any further questions.

I don't think so

You could do something like this:

for crontab in `ls /etc/cron.*/* /var/spool/cron/* /etc/crontab`
do
echo $crontab
cat $crontab
done