I think it should be safe to kill all PMM jobs and see what happens afterwards.
If the backup is still shown as "running" in Plesk after PMM jobs have been killed and no PMM jobs remain visible in "ps aux | grep PMM" command output, remove the entries from the "longtasks" table in the psa database. This should reset all. It will not delete any data, but it will interrupt running backups, so that these backups are corrupt or incomplete and need to be re-done.
P.s.: Check if the pigz jobs stop when the PMM jobs are stopped. If not, kill the pigz jobs, too. Before restarting anything, make sure that you have enough space in your temporary directory/partition and on your FTP backup space.