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.