How to measure the coil voltage in this circuit?
Of course you could use an isolation amplifier if you had a power supply on the isolated side so I'm guessing you don't.  For a passive measurement, you can get a measurement of the output coil by winding it bifilar with a sense coil.  The sense coil can be a different, lighter gauge, since it is not going to have any current.  This sense coil would have the same voltage as the secondary coil, were it not for the slight voltage drop due to the resistance of the secondary coil under load.  This means you may have to subtract the value of the resistive current loss, if it is significant.  You could do this by means of a table if you have a fixed load or possibly you can infer it from the primary current if you have a varying load.