You cannot (ever!) downgrade the ME firmware. The existing (running) firmware is responsible for the installation of updated firmware and will reject any attempts to downgrade this firmware. Further, because the update of the ME firmware is the first step in the overall BIOS update process, the rejection of the attempt to downgrade the ME firmware will typically cause the overall BIOS update process to be rejected (aborted) as well. Bottom line, you cannot downgrade the BIOS beyond the last BIOS version that upgraded the ME firmware.
Sorry, this is reality.