All the Vmc speeds are about flight with asymmetric thrust. The denser the air the greater the asymmetric thrust and the speeds will be higher.

Vmca is the minimum control speed in the air where with 5 degrees of bank towards the live engine (held with rudder to give minimum drag) that it is possible to maintain heading with a diversion of no more than 20 degrees off runway heading.

It does not mean that you can maintain altitude.

It is calculated at maximum take-off mass with the worst CG position.

So to the answers: The aircraft is not uncontrollable but straight flight cannot be maintained below Vmca.