Rotor current will be directly proportional to torque and stator current. In affect you could measure the stator current and calculate the approx rotor current. Measuring the rotor current directly with standard instruments will give errors as you are in affect measuring the current at whatever the slip frequency is.

A soft starter is in the circuit for so short a time, it is not likely that the fluting is coming from the variable frequency drive. My logic is that fluting is a low current long time event. Bearing damage that could occur from the very short and very infrequent duration of starting would have to be a very high energy (for that short time), and would more likely be pitting.

Side note: Soft starters will trip out if the motor speed has not reached at least 85% of nominal by the end of the start ramp cycle. So, that is why your soft starter is probably tripping. The soft starter should be giving you a "fault code", use the manual to verify why it is tripping.

Soft starters are allowed to work at its rated load in long-term, use the bypass contactor to short it after start finished, then we should pay attention to whether the motor has thermal overload protection in its circuit loop. In practical applications, if the working condition allows, we can use a soft starter to start several motors with the capacity bigger than the largest motor starting load, this will reduce investment costs significantly.