That is correct. We don't support, or allow, downgrading across versions of cPanel & WHM. Since 11.57 is a newer version than 11.56, you are unable to downgrade. That also means you cannot switch to a tier that currently is an older version.
Yes, that is correct. Note that cPanel 58 is tentatively scheduled for publication to the "Current" build tier next week if that factors into your decision.