Installation of an AC Drive on a Mixer - Help
Adding a V/F control system could be an approach. However with only V/F control the regulator can not see if the motor is acctually turning. 
One step further is adding an encoder to the system so the regulator can see that the motor is turning.
In both situations the available starting torque will be more or less 150% at low frequencies (3Hz)

A different approach could be OLV (Open loop vector control). Here the motor can realise a torque of about 200% of their rated torque at very low frequencies. Also here an encoder is possible turning the system into an CLV closed loop vector control. Here a torque control mode is possible. Thereby the motor is controlled at torque rather then motor speed. 

Try to understand the principles first and then look at your situation.The Closed loop vector control would have my preference for your mixer. For this approach you can look at the SINAMICS G120.

For your specific questions.


CLV can work in a standstill condition and would deliver enough torque during startup. You can compare this with a DC motor.
V/F might be enough but is not sure. For the other possibilties see above.
I do not think that energy savings can be achieved. The motor has to deliver its energy whatever the situation is.