1. We typically find default reference pulse widths of 1 ms for left, and 2 ms for rights used for the servos. In reality, the servos may need slightly adjusted pulse widths to reach their extreme rotation positions.
2. You are applying a pulse width that is trying to rotate your servo beyond its physical limits.
3. An H-bridge allows us to also change the rotation of a motor and apply a brake to quickly stop the motor spinning.
4. Many factors affect the reliability of braking, including the IC and your motor. You can adopt PWM-style braking as an alternative braking technique.
5. Vibrating but not turning is often the symptom of a mismatch between the coil energizing order and the coil stepping sequence. You need to identify and ensure the stepper motor's coils are connected correctly and match the stepping sequence. Consulting your stepper motor's datasheet is the best place to start.
6. The L293D has a voltage drop of around 2 volts, so your motor is only getting around 3...