You have database from 12.0.18, not from 12.5.30, it seems database rollback not required.
To remove Plesk 12.5.30 you should remove its packages, it's commonly named started from plesk-, psa- or sw-.
To gather list of packages run rpm -qa | grep '^plesk\|^sw-\|^psa-' then check that you don't remove something valueable, and remove package via yum.
It should not remove database or virutal hosts. Then install Plesk 12.0.18 via installed, but run it with --all-version option, to see all availiable releases, not last one