BLDC motor/PMSM parameters at reduced operating voltage
No, as long the driver can supply the rated current, the torque is the same. The resistance is pure loss, the current regulator has to apply more voltage, since the back EMF voltage at zero speed is zero, then it should be no problem to supply the rated current at start (n=0).
Yes, as the V_BEMF rises when motor is turning there is no longer possible to inject the current, when V_BEMF = Vcc
P = M * omega, torque remains as is, the speed (omega) is halved, so the power is halved.